Revit Building 9.1 is NOT supported on Vista. Vista came out well after Revit Building 9.1. I would wait until Autodesk software fully supports Vista. Right now, you're just jumping into unchartered territory.
As for 64-bit. All of the Revit products are 32-bit applications, not 64-bit. So I would recommend staying in the 32-bit realm for a little while longer.
Shobhit Baadkar
Director of Technical Development
CVIS, Inc.
Newport Beach / Fresno / Sacramento / Dublin