Actually - I realise I partially misinterpreted what you said; you are not trying to terminate the VPN tunnels via the LAN interface (though what I said about that holds true)
Only the IP addresses physically assigned to the MX WAN ports and/or the public IPs that they NAT behind are considered, when forming tunnels. VPN tunnels are formed based upon the public IP addresses of the MXs at each end.
If both MXs break out via the same Public IP, they form their tunnel between the physical IPs. If they have different Public IPs, they form between the two public IPs.
Your MPLS network will need to be able to break out to the Internet, as I mentioned in my previous reply.