We previously had one T1 internet interface handled by CPE router, and have just added a second "cable" interface (static) to handle the brunt of in-office internet traffic. After adding the second interface to the Untrust zone (in trust-vr) and giving it preferential routing, VPNs to the T1 no longer function.
My theory so far is that this is a routing problem, and although the VPN is bound to a specific interface the return traffic is being routed through the cable interface. (Which the cable modem probably discards.)
One theory is to put the internet interfaces in separate virtual routers, but that is based on the theory that the VPN code injects packets into the VR where its interface is bound - instead of just sending it out the interface bound to the VPN.
Any help welcome.