Virutal routers maintain separate forwarding tables. And are separate routers.
So if you have the peer in vtrust and your main clients in vuntrust you will need to create a connection where the traffic can reach each other.
this can be simply setting up a route with a next hop of the next vrouter or you can treat the vrouters as separate and connect two ports on the firewall together with one port in vtrust and the other port in vuntrust and setup a routed link between the vrouters.
But if there is no reason compelling you to have the peer in a separate router you should just setup the peer in the same vrouter as your other networks.