The post vise is the vise of choice for anyone doing blacksmithing. The purpose of the leg is to support the vise while hot metal is being formed while in the vise jaws (hammered w/2# smithing hammer or being twisted). A good bench vise is not made for taking real hits (even though the tail of the back jaw may be shaped like an anvil). A little hammering on stock in a bench vise is OK, but serious forging will probably break the rear casting.
The post vise has an design flaw that can be troublesome in some more exacting work. Because the movable jaw is anchored to the post leg, and pivots from there only, the 2 jaws are not really parallel (except on rare occasion). So if we are trying to do some fine work, the non parallel jaws can mar the piece being worked on, or be cause for the piece slipping.
I have found the old post vises to be made of quality steel, and have never seen one with a broken casting. Most of our import bench vises ($100 price range) are made from poor cast that is porous, so they paint some thick glop on them to hide the poor quality sand casting. I have an 8" brand name import with about a 1.5" diameter acme thread screw. When I really tightened it up (NO cheater bar, just a good snug) the screw stretched and would no longer thread into the nut. I got a new screw on warantee, but I have never seen this happen on any vintage post vise.
I see from some of the pictures a post vise mounted on a good looking workbench. You do not want to use the post vise for blacksmithing in such a neat environment. When hot iron is being formed, it gives off sparks of oxidized iron. These sparks can be big enough and hot enough to burn the wood bench, or bounce on the concrete floor, getting to something flammable. So the post vise is usually mounted to some well buried vertical wood post cut off to the height of the mounting plate when the portion of the leg having the ring is set into the ground so that the ring is just about even with ground. But as someone else mentioned, there should be a socket with a flat plate to set the leg in, and spread the forces out on the ground. It is preferred by blacksmiths to have a good tough dirt floor or some kind of compacted gravel for the floor of the smithy. And the anvil is usually mounted on a good hardwood round cut from a log, and it is quite near the forge and the post vise.
A good post vise is as important to the blacksmith as his/her anvil. (Yes, we have some excellent women smiths in our Blacksmithing Guild).
Paul in MN