I have a working 'Teleworker' PPTP VPN setup using the Windows XP VPN Client at one end and the Vigor 2600G at the other.

Everything works perfectly until I attempt to connect from somewhere using the same subnet as the Vigor, when I can no longer communicate with any of the remote servers.

To explain further: The Vigor subnet is set up as 192.168.1.x (the default). If I am on a remote site that uses a different subnet (say 192.168.200.x) and I then establish the VPN, everything works perfectly. I can connect to 192.168.1.1 (for example) through the VPN and all of my internet traffic goes through the VPN too.

But: If I am on a remote site that uses the same subnet as the Vigor (192.168.1.x) and I then establish the VPN, all of my internet traffic goes through the VPN still, but I can't connect to 192.168.1.1 on the remote site as this traffic never enters the tunnel, it stays within the subnet at the client end.

Is there a way I can ensure that when the VPN is established that ALL network traffic goes into the tunnel?

That's kind of a limitation of how networks work - because the remote VPN subnet is being seen as a local address, it never tries to go through the gateway, though you can set up static routes on the computer itself to fix that if you need to access specific addresses.
The best fix though is to change your 2600G's local subnet.