Now I'd like to try using sampled audio from Cubase 8.5 Pro as audio input to the vocoder, again using the microphone as the "modulator." I was thinking that I could route the audio from Cubase into the Montage and send that input through the Vocoder as part of the System Effects. I've been trying to set this up for a couple of hours, but haven't been able to get anything to work. The attached image shows how I have the system effects set up.

Hi Michael,
It is not clear exactly how you are returning audio from Cubase Pro to the A/D Input of the Montage.
Audio from the DAW typically is Output via USB and enters the Montage as a Digital Input, not as an Analog-to-Digital (A/D) Input.

You might want to take the audio track data and export it as a .wav
Then take that .wav and make it a Montage Waveform (import it to a Montage Drum Kit)
Create a Note-on event so that instead of being Audio in Cubase, the audio is triggered within a Montage Part
Now you can route that Part as the Modulator for the Vocoder.

While an A/D or internal Part can be the source for vocoding, the USB Digital Input is not available as a source.

Thanks for closing off that blind alley for me, Bad Mister.

Thanks to your advice, what I did was import my waveform into the Melas Waveform Editor, make a Motif XF Voice out of it, export it as a .X3A file, and load it into my Montage as a Library. Worked like a charm.

I think it would be useful if each of the inputs to the Vocoder (Source, Modulator) could come from a Cubase audio track (this would include using either one or both at once). If the Montage's hardware architecture could support it, this might be a cool feature for some future firmware update.