Not sure what you mean by the question. Yes, there is a VST for Motif XS/XF, Motif-Rack XS, S90 XS/S70 XS, MOX6/MOX8, MOXF6/MOXF8...
if by VST you are talking about Virtual Studio Technology (VST) as a way to address the hardware Synthesizers within the Cubase internal routing scheme; The Yamaha Editors for those products allow you to connect to the external synth hardware via USB or FW such that MIDI and audio are handled in a similar way to a software synth that is wholly contained on the computer.
Each of the listed products has an Editor VST, which sets up an Audio return path so that the audio can be routed to plugin effects within the DAW host, and is monitored via a “virtual” audio lane. Without this advanced VST routing scheme external hardware would not be able to be processed with software plugins in the same fashion as computer generated soft synths. VST is a routing scheme introduced by Steinberg in 1996 that integrates external instruments and external effect processors into the DAW/computer-based environment... specifically so that external devices could be routed, automated and controlled as if they were all “in the box”... Advanced routing and delay compensation algorithms make this all possible. (All of the advantages of being “in the box”, without the limitations of the computer CPU muscle, as the hardware synth has its own dedicated CPU) a soft synth with the muscle of any of the listed Yamaha hardware would certain be a heavy, heavy burden on your CPU that also needs to run the DAW, draw pictures on the screen, and juggle time stamping and all the other tasks you are doing. The external hardware is a co-processor, it brings its own CPU muscle to the party.
Take the Motif XS/XF or S-series synths as an example, you have 3-band EQ on every Part (all the time) to open a three band EQ on 16 computer tracks would put a strain on most computers running a DAW... You have that, plus a slew of boutique grade Insertion Effects, etc, available at all times in the synth hardware - adding 0 strain on your computer CPU. so again all the advantages of being “in the box” with none* of the CPU drain. *well very, very little of the drain.
If, by your question, you mean are there inexpensive software emulations of any of those Yamaha keyboard Synthesizers that work without being connected to the expensive Yamaha hardware, then, no. (Of course, not). The VST for these require the hardware synth.
Many people use the term VST or VSTi when referring to just soft synths... which is not correct. While there are soft synth that are VSTi, but by definition, a VSTi does not have to be software - when you route MIDI, and return audio via Steinberg’s innovative Virtual Studio Technology that is considered a “VSTi” also. Hardware VST
If you have Cubase Pro, you can setup EXTERNAL INSTRUMENT and/or EXTERNAL EFFECT routing manually for any external hardware that you own. (It requires “Pro” to do it without one of the Yamaha VST3 compatible Editors) If you have one of the other versions of Cubase, this advanced routing is handed within the Setup of the Yamaha Editor VST for the individual product in question.