Currently the virtual machine Fusion builds to support Boot Camp supports 32-bit machines. If you want to run Vista x64, you can manually change the guest operating system setting in the configuration file. Also the activation support needed to keep Windows from requiring re-activation when changing back and forth between environments is not there. If you run Vista Enterprise that is not a problem, for most retail copies of Vista it's a problem.
There's a more in-depth discussion including exact steps in this thread:
