Which VPN client software and version are you using? Or is this window PPTP vpn?
I had some problems with Cisco VPN Dialer client and the pasword manager function of the finger print reading software. Once I turned off the password manger function both the VPN dialer and finger print reader are happy to coexist. ( issue might not be problem if you are not using a RSA fob like I have to)
I am using both on my shift Cisco VPN for work and the Windows PPTP connection to reach my home network. So I can confirm that they both work from Vista and that windows PPTP works from WM6.
TMobile PPC -to I-Mate Jam -to HTC Advantage
HTC Shift -replaced my TC1100 tablet.
Still hoping the Shift will replace the Advantage as well, but need SDcard and GPS access.
If you haven't please donate to the HTC Shift Project