In response to my post regarding Vista on VPC, it was suggested that I bump up the ram and also install the Virtual Server 2005 additions (I had previously installed the Virtual PC additions and they had no effect)

I bumped the VPC ram up to 1gig (1024mb), and installed the Virtual Server 2005 extensions (odd that the server extensions would be recommended, but I did it anyway).

The end result was a very marginal performance increase, but some slightly increased functionality (the sidebar showed up when it hadn't previously - it must throttle down when memory is too low). However, Vista is still unusable in a VPC. I stand by my earlier recommendations.