Usage of Dual Ethernet Jacks (One for LAN, One for WAN)
Solution 1:
This can definitely be done using the route command. I wish I could offer exact advice on how to use it but I haven't used it in 14 years. it should be something like this
Set the default route to the first ethernet (assuming eth0) using the GUI, then:
route add -net 192.168.0.0 netmask 255.255.0.0 dev eth1
This assumes that eth1 is where the vpn traffic will go (I think)
To view the routes that you have set up use netstat -rn
This can get quite complicated very quickly. You might get a better answer if you ask at serverfault.com on how to use the route command.
If you really want to learn how to do this, it would be covered in TCP/IP Illustrated, Volume 1: The Protocols which is an excellent, very technical book.
Solution 2:
In newer versions of OSX you can turn on Internet Sharing from the System Preferences.
Update
After reading the poster's update, the above wouldn't work. You're looking at advanced routing for which I'd have to point you into the console. Open up Terminal and give these a read:
man routeman netstatman ifconfig
Beyond that, I don't know of a tool for doing this within the GUI outside of the OSX built in tools which don't allow for specific route addition.
Solution 3:
What is your VPN subnet?
if it is 10.246.0.0/16 for example, you could setup the default route through interface on jack #2.