Update: Make that THREE Prophet VS emulations. Via anonymous in the comments (guys and gals, start using your alias I can give you credit! : )
This Prophet VS emulator has been around for a while. That would make three!
http://manfreda.org/LordFrito/Prophet%20VS/index.html

Update: Trip! I just tried out the Auguar demo and the two envelops you can see in the picture below actually have a white dot that indicates where you are in the envelop while you hold a note down. Too cool.
Title link takes you to a post I was reading on Vintage Synth Explorer regarding a Prophet VS VSTi named
the Auguar. I decided to check it out remembering that I previously downloaded General Vibe's excellent Prophet VS emulation,
VectorSector. I remember reading that the people behind the VectorSector worked on the original Prophet VS, but I might be mistaken. If you know either way, please comment. I haven't been fortunate enough to get my hands on a real Prophet VS but both of these do sound good to my ears. They have that haunting quality the Prophet VS is known for. Click on the links for more info including samples and demos of each. I believe the Auguar is free while the VectorSector is $129 as of this post. It's amazing what you can get for relatively nothing today.
Auguar

VectorSector