Are you running Windows 2000 or NT at the VPN client? If so, attempt to connect to a shared resource directly by IP address rather than by name as follows:
net use *
If this works, then you know it is a name resolution issue. The most common cause for name resolution issues over a dial-up or VPN connection is the failure of the server to return a DHCP configuration packet containing the IP address of the WINS server. Verify this by making the VPN connection then running IPCONFIG /ALL. If there is no entry for WINS server, that's your problem.
Add the WINS server manually to the IP configuration of the VPN connection and that should solve the problem.
This was first published in February 2002