Brian,
You are on the right track, because the whole purpose of the leg on a blacksmith-style leg vise is to transmit the force of a hammer blow to the FLOOR, as opposed to the bench.
The weight of the vise should be on the leg. The attachment to the bench is just to keep it upright.
Whatever supports the leg must withstand the heaviest blow you intend to impart to the work. If you are going to wallop your work with a big hand sledge.......you can guess what would happen to a concrete floor.
Some smiths mount these on an upright timber embedded in the floor, with a second upright timber flush with the floor right in front of it. The leg goes into a socket drilled in the top of the timber that is flush with the floor. The end of this sunken timber was bound with a tight iron band. (Which may have been shrunken on like a wagon wheel's iron tire.)
The cross sections of these timbers is about the same as a RR tie.
The top of the taller upright has a thick horizontal hardwood plank just big enough to take the upper mounting plate of the vise. There are diagonal wooden reinforcing "cheeks" holding this plank to the top of the tall upright. The cheeks are through-bolted.
Saw this in the blacksmith shop at Allaire State Park - they have four vises set up this way, one for each forge. One could readily see that this would take whatever blow that the vise itself could withstand, and transmit that blow directly to the earth.
Just remember that the whole point of a leg vise is so you can "whale" on it without pounding your bench, or the concrete floor, to smithereens.
John Ruth