Mechanical soundness is what really counts. A little surface rust, flaking paint, caked on grease, etc don't really detract from the value.
Avoid cracks, big chunks taken off, badly bent handles, and missing pieces (lots of swivel vises seem to have lost their bases over the years). Unless it's a pretty common vise, replacement parts can be extremely hard to find and expensive if you do find them.
Poor and/or missing jaws are the tricky things. If they're just square, then they're fairly easy and inexpensive to replace (assuming they're not forged on) More complex profiles are problematic. I don't like to pay more than $.50/lb for any vise with jaw problems unless it's especially desirable (like the Monarch I picked up last month)
When all is said and done, I wouldn't worry too much about over paying a little for a good solid vise... an extra $10-$20 spent isn't that big a deal when you're buying something that's almost certainly going to outlast you.
Just a few questions if you don't mind. Is $1 a lb what the going rate for a vise that is in descent shape, i'm talking about a american forged vintage vise. something that needs a little love and paint to bring it back to life. How about a vise that needs to be completely taken apart and put back together, a complete overhaul $ ? i have no clue. Just looking for a little info to go on. at some point i will find a good one to bring back to life, today gave me encouragement.