Bolt a 4X4 to the workbench and attach it to that. Make a 6X6" plate from 1/4" steel, and weld a 1" nut in the center. This is the leg foot/shoe. Weld a 90 degree piece to the foot/shoe plate that can be bolted to the bottom of the 4X4.
If you have an open forge, put a 6X6 in concrete and use that as the mount. Stich a piece of 1" pipe in the concrete where the leg will sit.
Another popular method is to weld a plate to an old tire rim, and put the tire on filled with cement ( or use a 24X36" plate of decking steel, and stand on it while using the vise). Weld a piece of I-beam or 6" pipe to the plate and weld a flat "table" top to the I-beam or pipe. Drill the top plate for the vise and put a ring or nut on the bottom plate for the leg to sit in. These mounts can be moved as needed.
Obviously, all these take some planning on height to make the leg sit exactly in the foot on the bottom plate.