I have a user who is using TightVNC on Windows XP through Network Connect to access a second Windows XP system. If the second system has Cisco's VPN Client running, the connection will fail. The connection can be established with the VPN down but when it is started, the VNC connection is lost.
Forgot to add that this works fine internally.
i might not be getting the full picture here but if the 2nd pc is using cisco vpn than it is not on the same network as the first pc any more it is connected to the remote network and thus not allowing to connect is by design.