I had similar issues with VPN from a Vista SP1 client to Windows Server2003.
Could dial in and connect, even ping the server and some other network
workstation, could open web page devices (like router admin pages) get prompt
for login but receive no data. VNC would login in with blank screen, which
apparently was moving the mouse and GUI but could not see on the client. In
summary, VPN connected and live without proper large scale TCP/IP traffic.
- Disable ICS (Internet Connection Sharing) for the out going Vista client
connection (that the VPN uses), this one took me ages to find and was the
final step for me!
- Enable the MChapsV2
- Disable IPv6 (untick)
- Enable old registry entries for 40 and 56 bit compression if server
requires (search google VPN AllowPPTPWeakCrypto)
- Pull some more hair out...