I have setup MLP on 2600 and 3620 router with VPNs. Getting to to work is very dependant on the version of the IOS you are running. The first version of the IOS that I got it to work with (consistently)was 12.1(5)T7. If you are using the AIM-VPN module then :-(.
All you need to do is create an new MultiLink group and put the serial ports in the group.