I've got a vista guest OS running on an XP host.
You can choose it in the new virtual machine wizard. Give it some memory, mine's only 512MB but it installed fine. Runs good enough for some tests. If you have enough internal memory you could give it some more. It'll probably run better. My machine only has 1GB.

After I installed Vista in the VM I made a snapshot for a quick 'undo'. You can also shutdown the VM and make a copy of the disk image VMWare creates. You can easily create new VMs using copies of that image.