I’m pretty sure the answer is “No” but I thought I’d give it a try. Can anyone think of a way to Automate the On\Off function of a VST Instrument.
No you can’t (at least as far as I know).
Why do you want to do this—just out of curiousity?
I use Cubase for live performance as well as studio work. I was thinking it would be nice if I could turn off and on VSTs during the set in order to save CPU resources. For example, when song “A” startes the needed VSTs would activate… then at the end they’d deactivate … and so on…
Right.
Wow, you’re brave, using Cubase and VSTi live! Don’t you get a bit of a stutter when you turn a VSTi off, though?
No, not at all. Been doing it for YEARS with no issues. Started doing this with Cubase on my Atari ST. 
(Showing my age). I run it every week now at church for backing loops and its also my live rig. I’m completely softsynth.
Brilliant! Seems to me, you’d benefit greatly from having either a second rig running the VSTi, via SystemLink, or one of those dedicated outboard VSTi racks.
One way I’ve got around it is to just Freeze the tracks so the VSTis’ deactivate. I’ve also used MainStage a few times and it works well. Great program for $30. Even been demoing Ableton but so far it’s a mess hahaha. So I keep coming back to Cubase.
Yeah. I’m Cubase fo’ life!
So yeah, I think you should look at running a second rig—even a semi-crap PC—on SystemLink and use it to run a bunch of synths?