I've seen a lot of people stumped while trying to mount the Vista Beta1 iso image for thier VPC.  VPC gives some sort of error that the disc is no good.  Some people have figured out that if you actually burn the ISO image to DVD then mount the physical DVD, it works.  Since I don't have a DVD burner, I thought I would share with you another little trick.  If you have Daemon Tools, you can mount the image on your HOST machine using Daemon Tools, and then in the VPC attach it's CD/DVD drive to the "physical" Daemon Tools drive.  Hope this helps.