This would depend on the type of vpn and what you are trying to pass
if it is an ipsec tunnel (eroute) the traffic has to end up on the wan interface that ipsec is enabled on also the traffic has to match the source and destination of the eroute
if it is a gre over ipsec in this case a route has to be added pointing to the gre tunnel also return path has to be setup on the other end back to the tunnel.
if you have firewall turned on on the wan interface this has to allow the traffic in and out of the tunnel
there are a number of reasons that traffic will not flow
there are examples of setting up vpn tunnels in the document section of the transport product on the support pages of the web site