When Vegas Pro properties is set to 5.1, the only audio FX available in 5.1 are Track FX, Dither and Wave Hammer Surround - but these I understand are DirectX FX not VST.
Are there any specific multichannel VST plugins that you are thinking of using and if so do any of them offer a free trial period so that you can check them out in Vegas Pro.
Also, it might make a difference if the surround sound audio is 5.1 on just one audio track or split over 6 tracks - track 1 - FL; track 2- C; track 3 - FR and so on - the latter meaning that basic VST could be used individually on each component track.