Instead of trying to modify a cross sliding vise to use with a mortising machine, I went cheap and simple. The design uses a drill press vise and the total cost was under $10.
The heart of the design was a piece of heavy duty shelf bracket that was left over from some long forgotten project. I attached this parallel to the to the base by drilling and tapping. I used round head machine screws. A flat head machine screw was tapped into the base to keep the plywood sliding base level.(rail_attachment photo)
A piece of 3/4 plywood was grooved to accept the “legs” of the shelf bracket. The drill press vise is attached with threaded inserts and machine screws.(bottom_slot). One of those lucky things – the round head machine screws minimized the contact with plywood and it allowed it to freely slide from side to side yet maintain the parallel setting.
It may take a little longer to set up than a cross sliding vise but it works well and will cut smooth wall mortise.