The VPI is constructed to offer isolation to each shelf, but my concern is that the 331 might vibrate enough to over ride the design. I'm assuming that is why stand alone amp stands are used in other systems.
Give it a try,you have nothing to lose! I have used amp stands when my amp was located inbetween my speakers. I doubt very much that you would have any problem putting the amp on the bottom shelf of the rack. BTW,I never heard of an amp vibrating