A requirement for a WAN interface to become a transport interface (within VPN0) is to have reachability to the control plane!
In other words, if the Edge (cEdge/vEdge) router can't hit vBond and vSmart controllers by sourcing a given WAN interface, it won't use it as a transport interface!
Not that I recommend this to you, but the solution is to turn off "controll connections" fot that particular interface.
In your case, you can turn it off for your p2p links.