A few things to deal with here.

First, the "Vibraphone MotorOn" part and associated waveforms ("Motor Vibes mf" and "Motor Vibes f") have the vibrato recorded in the sample. The vibrato is not added after-the-fact with an LFO or other effects. So you're stuck with that vibrato for MotorOn ("MotorOn" PART which means "Motor Vibes mf" and "Motor Vibes f" - NOT meaning the entire performance called "Vibraphone MotorOn" - sticking to the waveform level) unless you can figure out a way to convincing use effects to compensate (flatten out the vibrato) then apply your own. This is going to be very difficult with the underlying sample recorded as-is - and you would probably be better off finding another preset and adding the vibrato with effects (use the "Vibraphone" preset - which sounds like no motor is running).

Yamaha's approach to vibrato on many instruments has pros and cons. On the positive side, sampled vibrato - assuming the vibrato is done in the "manner" which agrees with how you would have live studio musicians do it for your music is "always" going to sound better sampled rather than added as an effect. I say this because some of the nuance of instrument response for say a conical woodwind instrument, a string instrument (violin family, guitars, etc), or most any instrument will sound better and more convincing as a sample rather than as an effect or LFO. Certainly the range of expression in samples is greater than the current range of matching effects and LFO parameters one can use to try to reproduce a "natural" vibrato effect on a "dry" (not vibrato signal). On the negative side, and I have often run into this with an array of Yamaha sounds, is that I often want NO vibrato as an articulation choice. Here, the Montage does give non-vibrato options for many instruments. However, the balance seems to be about 5:1 (or "worse" - meaning less non-vibrato options) for vibrato:non-vibrato options per instrument (that I use). For some reason, guitar doesn't seem to have too much vibrato in most of the performances (also in Yamaha historically) - but acoustic instruments such as violins, woodwinds, etc - always have. Yamaha also coined "Sweet" (Like Sweet Tenor, Sweet ______ fill in the blank) to primarily mean that the sample is going to heave a heaping of vibrato. And I think this viewpoint is some legacy that brings us where we are. I would suggest that every single acoustic instrument that has a vibrato-recorded-in-sample waveform also have a non-vibrato version of the same player on the same instrument with the same room. Such that if we had 10 different vibrato waveforms or sets of waveforms (PARTs) - there would also be 10 matching non-vibrato PARTs. This would cover the vibraphone in there would be a MotorOff PART. At least a more "dry" or vibratoless option would allow for trying your luck at applying effects to achieve the vibrato rate and depth you want vs. using the one style offered by the preset available with vibrato. This still may not be as good as sampling many different vibrato depths and speeds - such that there is an array of options wide enough to fit most needs - but this approach would take up too much room (resources, memory) in my opinion.

Here is my approach to finding out if the "Vibraphone MotorOn" had recorded vibrato. There are other ways, but this is what I did:

1) Turned off all effects. With the touchscreen, touched the "FX" icon at the top of the screen and turned off all 3 effects. Some effects themselves are built to add this kind of effect, so I wanted to be sure to turn this all off.
2) Checked the LFOs. For the "Vibraphone MotorOn" performance - I noticed the Vibraphone PART (global) had no PART LFO set and also the element which was the vibraphone part (PART 1) did not have any element LFO (depth was 0 for all LFO destinations).
3) Verified motion sequence and arpeggios were off. The lights on the keyboard indicate the were off, but I double-checked also using the touchscreen.
4) Although not really able to accomplish vibrato without superknob automation (and I could see there was no superknob automation) - I still double-checked the Mod/Control for PART 1 and did not see anything in the assignable knobs - since the only setting would rely on insertion effects which were turned off in step 1. I did, out of curiosity, take a look at the other controllers (mod wheel, ribbon, aftertouch, pitch bend, assignable switches) and did not see anything interesting there. Mod wheel did have an effect on the release.
5) NOTE: the "Vibraphone MotorOn" PERFORMANCE - which includes two PARTs has a second PART which is NOT the "Vibraphone MotorOn" waveform. It's another waveform borrowed from the preset ROM and is labeled "CHROMATIC PERC". When you press the [ASSIGN 1] button (to turn it on), PART 1 (Vibraphone MotorOn) is switched OFF and PART 2 (CHROMATIC PERC) is switched ON. This is a non-vibrato sample and is likely related to the "Vibraphone" performance I suggested using earlier. It uses "Vibraphone 3" at different levels (ff, mf, and p). Your question focused on the waveform - and not the performance - but just wanted to give you more insight to the performance structure.

After running through this set of checks, I was sure the Vibraphone MotorOn had the vibrato recorded in the waveform's sample.

Perhaps a more direct approach would be to do the following - which is more in-line with your desire to audition parts:

1) Load up the Init Normal (AWM2) performance
2) Edit PART 1 (the only part), and edit Element 1. You should be looking at the "Osc/Tune" menu
3) On the touchscreen, touch the Name section (which is shown as "CF3 Stretch Sw St" by default for the AWM2 init performance)
4) On the left-hand side of the touchscreen, choose "Search" instead of "Number"
5) Narrow down the possibilities. For this, I would touch the magnifying glass section (upper right) and type the search phrase "vib" which covers "Vibraphone" and "Vibes" - which Montage uses both names for various vibraphone samples. Also switch the main category to "Chromatic Perc" to filter out all the other non-vibraphone "vib" waveforms.
6) Assuming you still have all effects turned off from the previous step (if not, for safe measure - I would turn them off - I don't think the Init performance does much in the way of effects to begin with) - you can scroll around each waveform and hear the different vibraphone samples pretty much in a vacuum (in isolation of anything else).

Hope this helps you discover some of these features by menu diving at your own pace for future performances/parts/waveforms.

As a follow on - to answer your "how to modify vibrato speed" question with more detail: for vibraphone, I think the tools are there to do a fairly stellar job using the method I outlined previously. Start with a "dry" (non vibrato) Vibraphone - and add your own using effects.

For this, what I did was first turned on all (3) effects toggles that I previously temporarily turned off for previewing the waveforms. Then, I took the "Vibraphone MotorOn" performance and started to modify PART 2's effects. Remember that PART 2 was the "CHROMATIC PERC" which was the non-vibrato ("dry"/"motor off") Vibraphone.

I took InsA - since it was already turned on and I wasn't concerned about compression (although you may be and would rather use InsB - Harmonic Enhancer by default) and changed it to what I thought would fit the bill: Tremelo. After having tried a few different things - I think the "Tremelo" effect with presets "Fast" or "Slow" are great starting points. You can modify the speed and depth under the settings for InsA to adjust to taste. "AM Depth" is amplitude aka volume and "PM Depth" is pitch. PM Depth is set to 0 which is fine - although there is a bit of a doppler effect in a vibraphone - I think the amplitude modulation really does the trick without having to deal/mess with the pitch. I'm not a percussionist nor am I a purist when it comes to this sort of thing (at least not for this instrument) - so you can dig deeper into the settings if you wish.

I have tried some of the other effects. Auto pan sounded too much like the Rhodes pan which used light bulbs - which is great for that - but wasn't quite "the sound" for me. Rotary speaker didn't cut it for me either. Although - you do have an array of choices here including the "plain jane" LFOs.

Remember to "turn on" PART 2 (which is what I am suggesting editing the effects for) that [ASSIGN 1] button should be brightly lit and pressed if not. From the [PERFORMANCE] (HOME) screen, you can see pressing a key and switching back and forth between [ASSIGN 1] ON and [ASSIGN 1] OFF will either show PART 1 or PART 2 responding. And we want PART 2 to respond.

... I've been playing more on the PART 2 (CHROMATIC PERC) modified PART and don't like as much as the PART 1 (Vibraphone MotorOn). It's not due to the programming I suggested - it's due to a difference in both of these PARTs.

Notice the Vibraphone MotorOn uses the following waveforms: "Motor Vibes mf" and "Motor Vibes f"
and the CHROMATIC PERC uses the following waveforms: "Vibraphone 3 p", "Vibraphone 3 mf", and "Vibraphone 3 ff"

Notice the big difference here is the PART 2 (CHROMATIC PERC) has a "p" waveform and the original sound you like (Motor Vibes) does not.

What I wasn't liking was when playing soft - there was no real attack. Which may be fine - but it's "too different" from the original "Motor Vibes" which you like so much. Maybe you really like it better for more dynamic expression. However, the last edit I would do would be to adjust PART 2 such that the "Vibraphone 3 p" waveform is not used and instead to map the velocity range of "Vibraphone 3 mf" to cover the p-mf range. This would give the PART 2 with adjustable vibrato more "bite" to match the Motor Vibes sound.

Not really related to how to add vibrato at a custom speed - but still part of the big picture to get you closer to the original goal.

I must say that I also love the new Vibraphone performance in the Montage.
It's highly playable and beautifully dynamic for jazz gigs.

but, I have to agree with you as I've been looking for a MotorOff sound as well.
maybe it's something Yamaha will add in a future update?

Reread above - it's there. Press the [ASSIGN 1] button located to the right of your mod wheel.

No need for an update for this particular one.

And as I mentioned - the "motor off" sound that's already a part of the "Vibraphone MotorOn" performance is even more dynamic in terms of articulation because the "motor off" version has a softer mallet sound then the "motor on" sound when playing with low velocity.

By the way - I did mention this - but in more vibe terms. If you want to accomplish a flutter pedaling or slide dampening (halfway muting the notes which takes out some of the high harmonics, softens the attack, and causes the notes to release quicker - not sustain out) then move the modwheel "up" (make the modwheel detent move closer to the back of the keyboard - or further away from the ribbon).

Somewhat of a performance edit - if I were to change one of the "intent" parts of the original preset performance, I'd reduce the pitch bend range from +/- 1 whole step to +/- one half step. The whole step range doesn't sound natural to me - and the max bend range used on vibes is generally a half step. Although there are exceptions to this - and it may depend on what you're trying to do - I would still limit this and feel comfortable that I'd have a limited bend range in the off chance I wanted to bend the note. Outside of this range doesn't sound right to me in part due to some limitations of how this (pitch bend) is accomplished.

The "MotorOn" sound of a vibraphone has a natural built-in movement, so indeed, it apart of the waveform. Via XA CONTROL (ASSIGN SW 1) you can switch to a vibraphone set of Waveforms sans the motor. Things like PB range fall into science fiction as the acoustic instrument cannot, period.

I think there was a way to audition built-in waveforms on the Motif XF. Is there a way to do this on the Montage?

Yes. From the HOME screen
Press [EDIT]
Press [PART SELECT x] to select the Part
Using the Element items, 1-8, along the bottom of the screen or the third row of right front panel buttons, 1-8 select the Element.
Touch "Osc/Tune"
Highlight the Waveform area and press [Category Search]

BM,

You are are not aware of extended techniques for the vibraphone. There's even a Montage vibraphone performance which uses the ribbon for pitch bend which is a more natural control considering you would do the same action with your hand or mallet to produce the pitch bend - so I believe the programmer tasked with this preset has the right idea.

Check it out:

https://www.youtube.com/watch?v=MX_Er8nJ2Ok

It's similar to what you would do on a drum head to get a similar type of pitch bend.

Even the tonewheel organ does pitch bends if you cut off the power - and some players use this intentionally to do pitch bends in either direction (down and back up). You're probably aware of this technique. Just trying to illustrate a connection between a couple different instruments and extended techniques that go beyond what some think are inherent limitations of the instruments.

So my programming tweak was to get the PB to sound more natural from an acoustic instrument reference.

I also pitch bend my trumpet tone constantly. I mean my real acoustic trumpet. All acoustic instruments can pitch bend. Not sure what he means by that.