This may help. quoted from
Can't access the internal server when remote client establishes VPN
Symptoms: Two offices are connected using a vpn. Both offices run W2K servers, RAS & TS. When office A connects to Office B you can not connect to office A from anywhere except from office B. You can not connect using TS, or a vpn connection. In order to gain access to office A, you have to connect to office B, disconnect the client in RAS, then you can connect to office A.
Resolution: When establishing VPN to the office A, the routing table changes. To fix this issue, disable the "Use default gateway" on the Office A VPN server. Or modify the routing table manually.
Robert Lin, MS-MVP, MCSE & CNE
Windows, Network, Internet, VPN, Routing and How to at