Once you assign the VPN interface – Interfaces > (assign), add the interface, then enable it with an IP type of 'none' and restart that VPN instance -- you'll get a gateway entry automatically. Once you have the gateway entry, you can craft a firewall rule to catch the traffic from the IP you want and force it to use that VPN gateway.
You will have to make sure that NAT will apply to that traffic, either locally as it leaves the VPN interface, or remotely at the other side of the VPN, or it won't make it out to the Internet.