They are one and the same. The full name of the program is “Yamaha S90 XS/S70 XS Editor Standalone/VST”. The program can be launched by directly launching the program icon, or can run inside a VST3 compatible DAW software, like Cubase, where it functions as a VST Instrument plug-in.

This VST Instrument launch is done within Cubase and establishes advanced routing capabilities... allowing your external hardware (S-series synth) to return audio in the similar fashion to how soft synth plug-Ins do. Meaning you can process the returning audio with plug-in Effects, you can Freeze audio in temporary audio files —which allows you to open multiple instances of the synth in any Project, you can render audio, in real-time, using the Export Audio Mixdown.

What likely has happened is you’ve updated your Cubase version and it has lost track of your plug-Ins... or your version of Cubase is older and the plug-in you’re trying to run is no longer compliant with the version Editor you are trying to run; you need to navigate to the place where Cubase keeps its links to plug-Ins an Refresh it... make sure it is not on the ‘blacklist’. If it is either reestablish the link or update the version as instructed.

You’re looking for the “VST Plugin Manager” ... Go to STUDIO > VST PLUG-IN MANAGER (in older versions look under “DEVICES” > “Plug-in Information” > “Program Plug-ins”
The S90 XS/S70 XS Editor VST should be listed.

You might consider updating your Cubase version... as more things will become non-compliant. You may want to check with Steinberg through your MySteinberg Account about functions are no longer supported. (Cubase 5 is more than a decade old).

Let us know.

Edit Update: I just found an old copy of Cubase 5 (for Windows), and was able to launch the Yamaha S90 XS/S70XS EDITOR VST ... so check on refreshing your plugins...

Sorry for the delayed response. I installed a new router and modem over the weekend and my home network had a nervous breakdown.

The biggest problem I have with this is that I don't know what a working configuration looks like. I keep trying random things, moving .vst3 and .dll files around, installing/reinstalling software... it would really be a help if I knew which file links the S70xs VST to Cubase 5, where does that file need to be, and where can I download a known-good copy?

https://usa.yamaha.com/products/music_production/synthesizers/s90_xs_s70_xs/downloads.html

Use: S90 XS/S70 XS Editor Standalone / VST V1.6.8 for Win

... and this: S90 XS/S70 XS Editor VST Installation Guide (V1.5.1 or later)

for directions.

The guide states Cubase 5.1.1 is a minimum requirement.

To use the S90 XS/S70 XS Editor VST as a VST3 plug-in of the DAW application, you’ll need to install the following
software programs. Follow the appropriate instructions below corresponding to the software you’re using.

Driver Installation
To operate your Yamaha hardware from your computer via a USB cable, you’ll need to install the USB-MIDI driver.
The USB-MIDI driver is necessary when using a USB connection to transfer MIDI data back and forth between your
computer’s DAW application and the S90 XS/S70 XS.

S90 XS/S70 XS Editor VST cannot be used over a MIDI connection.
Make sure to properly install an audio device when connecting the S90 XS/S70 XS instrument to the computer via USB cable. For more
details, refer to the section “Audio Connection to the Computer” (page 5).

S90 XS/S70 XS Editor VST Installation
Follow the instructions below to install the S90 XS/S70 XS Editor VST programs.
Windows
1 After the downloaded compressed file is properly extracted, double-click the “setup.exe” file.
The S90 XS/S70 XS Editor VST setup dialog appears.
2 Follow the on-screen instructions to install the software.
... Mac instrucions deleted ...
When the S90 XS/S70 XS Editor VST and the other necessary software is installed, you can start the S90 XS/S70 XS
Editor VST. You’ll need to start the S90 XS/S70 XS Editor VST from the VST Instrument window of DAW application.
For details about starting and setting up the DAW application, refer to the manual of the DAW application.

... even though this is for Mac - it shows what the VST will look like in Cubase and some general usage tips - not really setup steps:

https://www.youtube.com/watch?v=hh-X6v3FMKg

... the "s90xs_editor_vst_en_om" PDF has some instructions on how to setup the editor VST within Cubase.

I've already installed the USB driver (3.1.4) and the Standalone editor 1.6.8. The standalone editor sees the S70xs and is fully functional, i.e. data transfers work from and to the S70xs, patch editing works, etc. (Attachment 1) which seems to validate whichever VST file the SA editor uses.

Cubase sees the S70xs hardware and makes it available for channel assignment (Attachment 4), but the VST doesn't show up in the Instruments panel (Attachments 1 & 2).

Cubase has been de-installed and re-installed, the USB driver and SA editor de-installed and re-installed multiple times and the PDF's have been read. What I'm looking for now is some picture of what a working installation looks like. These installers don't give a clue what they've done. They just run and quit so I have no idea, for instance, where the VST was installed and what it's called. If I knew that, I could experiment with placing a copy into the various locations Cubase seems to scan when it starts up (or knowing precisely where the VST file should be placed for Cubase would be another help.)

Again, the editor installs both the Standalone and the VST. Above is how the Port Setup looks like when you successfully launched the VST Editor inside of Cubase.

We need to determine why in your second screenshot you do not see EXTERNAL. — that is how you launch the VST Editor... when you launch the program by *selecting* VST Instrument > click in an empty slot > External > , the program launches as a VSTi

Please download and install the “S90 XS/S70 XS Remote Tools V1.1.8 for Windows”

Since you’re using Cubase 5 ...we have to consider whether you're running or can run 64-bit or 32-bit versions and whether Cubase 5 supports the versions you have installed. Sorry off the top of my head I don’t recall what the Windows situation was with 32/64 bit versions and what the consequences were if not compatible.

You can check with MySteinberg Account what Cubase 5 is capable of running and not running (it was more than a decade ago so we must consider all these things...) let us know if installing the Remote Tools (which includes the Yamaha Extensions) makes any difference.

Did you look at your “Plug-in Information” ?

Thanks, but I'm going to give up on this. I've used backup images to restore my PC to multiple points in the past two years when I absolutely had a working S70xs/Cubase link, and the results have been the same in each case. Existing Cubase projects show the S70xs VST as missing, new projects don't referenece it at all. Damndest thing I ever saw, but I'm going to have to do with out a DAW or maybe look at open source software.

Thanks again for your time and effort.

I did some additional research on your issue...
The Editor version is stated on the download page as NON-compliant. So you will not succeed with the latest version of the Editor software. You can remain frustrated or you can update your version of Cubase. Cubase 5 is eleven years ago (that’s a millennium for software)... seems you’ d need to find a very old version of the S90 XS/S70 XS Editor in order for it to work. The other option is downgrade your version of the Editor to one that has been tested to work on your ancient version of Cubase...!

I was able to find out about when Cubase 5 became NON-compliant... it was quite a while ago - more than 5 years ago.
You’ll need a version much earlier than the current V1.6.8.. I highly recommend you try V1.4.0 or V1.3.0 as they are the versions that report they work with your eleven year old version of Cubase5. Both 32-bit and 64-bit versions of V1.4.0 are still available on the S90 XS/S70 XS official download page (and now I know why!!)

From the Editor Download Page....
V1.6.3 to V1.6.8
Now supports Cubase 9.5 series.
Now Cubase 6, 7, 8 series are NON-compliant.
Solved a problem in which the application might be forced to quit occasionally when using the 64 bit PC.
Fixed some minor problems.

From the earlier posted version
V1.6.0 to V1.6.3
Now supports Cubase 7.5.
Now supports Windows 8.1.
Now Cubase5 series are NONcompliant.
Now Windows XP and Windows Vista are NONcompliant.
Fixed some minor problems.

From the earlier version
V1.3.0 to V1.4.0
[New Features]
- Now supports Cubase 5.5 series.

[Fixed Bugs]
- Solved a problem in which the save operation might fail when you select "Save As..." after selecting "New" from the File menu.
- Solved a problem in which Cubase is forced to quit when you close the Editor window during the Import operation when in the ONLINE status.
- Solved a problem in which Cubase might be forced to quit occasionally when MIDI data is played back with the Editor window open.
- Solved a problem in which the VSTi settings of the Editor might not be restored when opening a Project file of Cubase.
- Solved a problem in which the entire volume might be lowered to 0 when changing the mode from the Editor.
- Solved a problem in which Cubase is forced to quit when clicking the close box of the Editor window repeatedly.
- Minor bug fixes.

Uninstall V1.6.8 then install V1.40
Hope that helps...

Version 1.4.0 of the editor did fix the problem! Thanks so much for your help, Bad Mister!

Thanks for the question, it caused me to look into the version histories. Glad it solved the problem!