I have both GS 3 Orchestra and GVI 3. I use GVI more because I like using the VST plugin instruments with Cubase, better than the Rewire.
But if I update to GS 4, do I still need to update GVI to 4 to use VST?
Nope, the whole family of gig products after and including v.3 can peacefully co-exist on the same machine. Meaning, you can have GS3+GS4+GVI3 (+GVI4 if you want), and use each one for it's own designs. There would be some redundancy in this case though, since either app's 4th generation can do all that v.3 can do and more. But the same applies to your case: Install GS4, and your GVI3 installation will survive, and you can use it as a VSTi all you want.
GS160 2.54 is used for sample playback in a dance review group of mine.
GS3 is now a LSAD Drum DAW.
GVI is going to become my personal DAW for live and recording, w/ GS4 hosting all of my GVI / Kontackt based instruments.
I have used the 160 2.54 DAW for years after upgrading from Gigasampler.
All DAW's are using Scope DSP cards w/ 100% stability.
I use to have 2 DAW's onstage just in case. after several weeks w/o a hiccup I started using both to load everything. Then GVI came along and out loaded GS3. Since these DAW's never crash, I only use one, but have Creamware ASB's, and hardware analog synths for redundancy.
Scope and Gigastudio have, and will continue to put cash in my pockets.