Is this the stamped steel pan at the bottom? If you can't get it out with vice grips, you might just want to take it off. That way you can work on it on the bench, with a cool glass of suds by your side. Run a series of standard drill bits on the bolt FROM THE INSIDE, so the cutting action of the drill bit tends to screw the bolt out rather than in.(grind the head off first if you're doing this). Even if you miss a little, you can retap to a slightly larger size, and be positive you left no metal chips inside