I have installed on both 32 and 64 bit (on same system, dual boot method). Previously (and still installed) Vegas 7e [on both].
Installed v8 on both - no problems.
The crunch time - performance differences.. Using a pure multiple generated media test project file (the same on project on both "systems") with same render settings etc.
Render time for 32bit. 3m02s
Render time for 64bit. 3m02s.
Result: Suprise for some perhaps, less so for others.
The benefit to me: 64bit handles more memory - so Vegas is effectively using up less of my RAM (as a %, not allocation in MB before anyone starts... - 32bit ran up to 73%, 64bit went to 67%). Both were running CPU at 100% for the whole render time.
HTH some people looking for a comparison
Ken