been there done that... if you can get a really tight squeeze with your vice grips...(may need a second person to help) you should be ok.
another way is to lay the bike down, in good light...VERY CAREFULLY...use a dremel with a cutoff wheel, and cut the flange of the bolt enough so the pieces with snap off. Once you relieve this pressure it should come out like nothing... be careful if you notch your case you could quickly turn things into scrap metal. if your lucky you'll have a washer around the bolt that will help prevent this.
when you get the new bolt, there is a soft gasket that goes in there. those plugs do not have to be that tight!