Another neat base for a post vise is a tire and rim.
Leave the tire on the rim and let out the air (don't pop the bead). Drill a 1" hole in the tire where the post will sit (do some trial fitting).Cut a 6" slit in the side wall about half way between the rim and the tread. Spread it open with a wooden board. Fill the tire with concrete, and remove the board to let the slit close.Shove a 6" piece of 1" pipe in the hole you drilled,leaving about 1" sticking out (the foot for the post leg) After the concrete drys, weld a section of pipe (3" IIRC) in the rim center hole. Weld on a top plate and attach the vise. To move it just tip it on its side and roll it to where you need it. It won't tear up the garage floor or the grass. If you need to put the "boot" on it, you have a nice tire surface to brace against. The concrete filled tire makes a good counter weighted base. The same base can be made to mount other tools that you want to make portable, like a disc sander, a post anvil, or a forge.
Stacy