Enable RIPv2 on the menu NETWORK/Router for your VPN00 interface and any other LAN interfaces on both zeroshell routers where your servers are connected.
Verify both zeroshell routers hear the RIP announcements under menu NETWORK/Router
Learned by RIP
Destination Netmask Type Metric Gateway Learned From State Time
If you see all the servers subnets in the above menu then the zeroshell routers know how to direct traffic to your servers. You can then add a virtual server entry for your servers on either zeroshell router.