The VPN software won't run because it requires a driver to operate. Cisco only has a 32-bit driver.
While you can run 32-bit programs on a 64-bit OS, drivers have to be written & compiled specifically for the architecture they're running on.
You easily multiboot x86 and x64 Vista on the same box (or any other combo of Windows really). The rule of order is to load them oldest to newest, so 2000, XP, Vista, etc.
I would say +1 on the suggestion to use VPC/VMWare. This is how I run all my VPN clients.