If the upload worked correctly, here is a version that I made some time ago. It is made with plain old fir and I used cheap pipe clamps. If you look closely, you can see that there is a 2x2 clamped in the main workbench vise - it has two 3/4" dowels that extend up into the front of the legs. That's what secures it into position. When stored, that 2x2 is flipped over and stored in the top of the same two dowel holes in the legs. It has proven nice and sturdy and handy.
I've been asked whether or not the clamps stick out too much. I leave them on the front for convenience, but if they get in the way, I reverse the clamps. Also, if I need more than the 4" gap the vise provides, I can use longer pipes for whatever depth I need. Also, I have pulled the clamps out to use elsewhere temporarily. Versatile setup.
Mike Cebula