Should be possible to do it via a switch.
This will work straight away when you ensure that the physical ports that connect to the metro-WAN are set as access ports. Setting the ports to access mode causes them to send traffic untagged so vlans have little significance. This will be a routed link and as there will be just two nodes, you could use a /30 mask.
If you want tagged frames, (your WAN must accept them as well!!) the vlan id on both ends must match.
CDP is best disabled on these ports.
VTP is no big deal either as you are using different domains, use a vtp password for extra safety.
regards,
Leo