Since it isn't obvious, I'll ask!

I’m not sure how they are traditionaly done, but I would use a big tenon on the table and a slot on your movable clamp.

-- Gary, East TX -- The longest journey begins with a single step.

I saw plans for one like this recently, and the bottom just slid on a piece of 3/4” pipe to keep it aligned.

-- Charlie M. "Woodworking - patience = firewood"

I think what you are wondering about is the parallel guide. It is in the Autumn issue of woodworkingmagazine. It may be on the website in letters. IOt ia pictures as a mortised piece of wood with holes in it to put a peg in depending on the opening of the vise jaws. Christopher Schwarz said it could be at floor or 20 inches from benchtop.

-- WOOD/DON (...one has the right to ones opinion but not the right to ones own facts...)

I was going to say what Don said. Cut a mortise in the leg, near the bottom. Then attach a long-ish thin (maybe 7/8” wide by 3” high?) board to the bottom of the leg that will fit into the mortise, and that has holes drilled at intervals. Get the vise pretty close to clamping distance and slip a peg in the hole just outside the leg. When you crank the vise, that peg will keep the bottom of the leg out and increase clamping pressure up top.

Don’t know if that was clear or not, but should get you close anyway! I haven’t built mine yet, but plan to, so if there is a flaw in my explanation, feel free to correct me!

-- Eric at http://adventuresinwoodworking.com