VPN through proxy servers

I have 2 domains that each connect to the internet via proxy server over cable modems. I’d like to establish a VPN using PPTP and RRAS between them. I’ve read all the docs and newsgroup posts, and still no luck. My headach comes from the fact that when Itry to add a PPTP interface in RRAS, I get a message saying “WAN routing is required to add and interface”.
The Proxy servers are also the PDCs and the RRAS all at the same time. I’ve got packet filtering set, with certain exceptions defined for both sides, including the inclusion of the PPTP Call and Recieve, and NETBIOS exception for the remote IP address.
It’s a complex environment, I know, but I’m bound by the customer’s requirements.