With your current network configuration all clients will send packets to networks they do not know to your main router. I do not know what your main router is, but either that has to route packets based on a policy to route them to your AR750 or these clients need their default gateway adjusted.
Either method will cause them to use your AR750 as gateway.
After that it might not actually require any special configuration to make the AR750 put the traffic into the VPN for you. Possibly the “force VPN” option can stay on. The first problem you have at the moment is splitting these 2 packet flows from clients that should and shouldn’t use the VPN.