Router on Different Subnets

Below is the architecture of how my home network is setup:

http://i.imgur.com/RCuYWhk.png

I looked around and this was the closest solution I could find:

http://unix.stackexchange.com/questions/9087/how-can-i-use-synergy-when-connected-through-different-routers

Do I do the port forwarding on the VPN Router or the Cable Modem? Currently I did it on the Cable Modem but it still doesn't work.

Any ideas?