Don't know what's happening with your VPN. I CAN tell you that I just tried my PPTP VPN (hosted on SBS 2003) from my Vista Ultimate test box and that one's working fine. It connected right up and I can browse shared folders.
What kind of VPN Server are you using?
Are you using the built-in Vista VPN client, or a 3rd-party client?
PPTP, L2TP, or ?