Well, after much gnashing of teeth I finally was able to get my PPTP VPN connection to work again on Windows Vista. I've been unable to connect to my office for over a month now. Turns out there is a conflict somewhere between the standard network adapter and the network adapters that are installed with Virtual PC 2007. I've been working with Janani Vasudevan and Aanand Ramachandran of the Routing and Remote Access (RRAS) team.
It turns out that my connection was only attempting over an L2TP connection. The final resolution was to uninstall VPC 2007, reboot and reinstall. After doing this it resolved my problem. If I had to do it again, I would probably try uninstalling the Virtual Machine Network Services from my network adapters, and reinstalling them prior to uninstalling VPC. It probably would have saved a reboot or two.