InterVlan Routing to a VPN

We have a Third Party that would like to allow us access to a subnet on their system via a Site to Site VPN.

There is no need for them to access stuff on our network but they want us to use a small subnet to avoid clashes on their end of the network ( 192.168.255.1 / 24 as an example ) we have set this subnet up as a Vlan and have setup and established a IPSEC Tunnel and the tunnel works if your on aforementioned subnet. 

Is there anyway to get a Fortigate FG100 to route traffic from another subnet over this tunnel? I can't create a static or policy route to route traffic to the gateway address 192.168.255.1 as it just complains it's a interface address (well yes )) 

Essentially we want it to take traffic from our vlan(s) and act as a NAT gateway sending stuff over the VPN.

In the past we have done this by having another router take traffic out of the main router and pipe it back in via a WAN port. This is a little Jank though and I was hoping for something a bit more elegant.