Can I install windows 7 x64 in a VPC where the host is vista x86?

You can install a 64bit guest OS on a 32bit host if your hardware is x64 compatible, and your virtualization software supports it. At least VirtualBox does, but I believe Virtual PC does not.

Some hypervisors (VirtualBox and some versions of VMware--maybe also others) do support 64-bit guests on 32-bit hosts, as long as your CPU is 64-bit, supports Intel VT or AMD-V, and has VT or AMD-V enabled: serverfault.com/questions/52442/can-an-x86-host-run-x64-guests