[b]DO NOT[/b] remove the pins with the upper, or barrel, in a vise. That is a sure way to bend the barrel, I've seen it done more than once. Besides that the barrel acts like a lever and absorbs most of the hammer's impact.
Lay the barrel on your bench and support the off side of the front sight base with something solid but malleable. I use nylon blocks 1/2" thick. Some use a chunk of wood. Find the biggest pin punch (mine is 1/4" diameter steel) you can and use it. My little 8 oz ball peen has never failed me on hundreds of front sight base removals.