Just trying to understand the situation. This is not possible if you are trying to use it as a vpn server. The setup does not allow for two gateway IPs on the same nic. I would setup the server as a regular server/gateway.
You can setup the server as a server only setup and use it as a vpn server, but it would give you an IP in the range of the wan address.
Sorry, I guess I don't understand why you would want to do this.