If I understand your concerns about the expansion table top correctly, I think there is a solution. This would work if you have some good quartersawn or stabile hardwood (maple, maybe?).
Make a frame of hardwood. The frame would have the end laminated to accept the Veritas vise. The inside of the frame would have a step cut into it to accomodate the MDF or your choice. I would make a substantial step, perhaps 1/2". You could also attach cleats to the inside of the frame. The frame should be cross braced beneath, at intervals, to help support the router placement and to insure against movement. You can glue the MDF into the frame, fasten, or let it float. I would do the first two. Irregardless, this method allows for any movement of the MDF and allows for the vise mounting.
Sawdust is not dirt