You can NAT the VPN traffic on the PIX.
The configuration will depend on which interface is the MPLS connection.
For example, if the VPN clients terminate on the outside interface, and the MPLS network is reached via the inside interface, then you:
nat (outside) 1 192.168.16.0 255.255.255.0 outside
global (inside) 1 interface
The configuration will change depending on the interfaces.
If both VPN and MPLS are reachable via the outside interface, you can reroute traffic back out the same interface (u-turn) if running 7.x or higher code.