I don't know anything about that vise but perhaps the softer brass was deformed and is now locked into irregularities in the casting. In this case heating or lube would likely not work. Broken bolts/screws are usually backed out by rotating after drilling a hole to accept an extractor. Seems like this would have to be driven out (which could deform the metal more) or cut out.

Without seeing it, perhaps drill a hole, tap threads on it for a bolt, and devise a puller to try to extract it. If that didn't work, I'd consider drilling a number of holes to remove most of the volume then gently chisel and/or cut away inside (Dremel with carbide bit?) until the brass is thin enough to be pried away from the inside walls. (Assuming the guess about the deformation is correct)

If I could see it some other idea might present itself. Perhaps consulting with a local friend who works with metals would be useful.

JKJ (not an expert but do dabble in metalworking)