In VPC its unbearable. Dont know about VMWare
I run Vista on VMware Workstation 2.5.2 and ESX 3.0, and recently moved to Workstation 6 beta (much better!).
Give it at least 1Gb of RAM-2Gb is better, make sure that your host is at least 2.8Ghz-HT or dual cores are better and have the fastest hard drives-SCSI U320 15K RPMs work wonders, SATA is OK, and is better than PATA, but SCSI is best for virtual, not just for Vista but for running most OSes in virtual. I try to keep competing VMs on separate drives to keep contention low which is a best practice.
My system has 4Gb of RAM and can go to 16Gb and has dual CPUs and that is pretty important if want to run more than a couple of VMs so look for that if you are looking for a new host at some point. I usually have three or four running depending on what I'm doing. My host OS is CentOS 4.4 so that may be another reason Vista runs almost as fast as bare metal and is yet another reason to stay away from MS Virtual PC and Server.
On ESX 3.0, I'm hard pressed to tell a difference between native and VM in doing most tasks. That's because ESX is bare metal virtualization.
The Aero interface won't be available due to lack of 3-D graphics. Having seen it and worked with it, it's cool, but after a bit, yawn...