PrPro can use many VST's beyond just the ones in its folder structure. It uses many from AE. Normally, one of two things happens:
1.) the VST is fine and is used in PrPro
2.) the VST cannot be used by PrPro, and the very first time that it is launched, it will tell you this in a little dialog box. It'll still load them with no problems and no more dialog boxes.
However, there is that thrid situation. Not only can PrPro NOT use the VST, but it crashes the program. Curt has addressed that. Besides Cubase, Native Instruments also has VST's that kill PrPro, at least in earlier versions.
Follow Curt's link for some more details.
thanks then guys