FRC, Routing, VPNs, and Subnets

[linuxboy]

Hi all,
I have a very interesting networking issue that you all may be interested in taking a crack at. I have 3 subnets at my house (which is where the robot currently resides). The first is 192.168.1.0/24 it is the network on which all the normal devices in my house operate (everyone's computers and mobile devices). I then have a virtual subnet 192.168.10.0/24 on which VPN clients get assigned IP addresses. Lastly I have the robot subnet 10.37.80.0/24 on which all the robot stuff connects (this is done to keep the robot network clear of extraneous broadcasts from all the other devices). Pinging any device on any subnet works fine from the other subnet (ICMP requests are all routed properly), as well as TCP as far as I can tell. The issue occurs when trying to receive UDP packets on the VPN subnet from the robot. Using tcpdump I figured out that the UDP packets from the DS are successfully routed to the physical network, and the responses are successfully routed to the VPN gateway, however, they do not reach the virtual network that is 192.168.10.0/24
The VPN gateway is running linux and routing is done with iptables.
The INPUT and FORWARD chains both have a policy of accept and the nat table is empty.

Anybody have any idea what I'm doing wrong with the routing on the linux box? If people want raw output from tcpdump or something I'm happy to provide it.

Thanks,
Oliver