If the bolt is still in there but you can't get vise grips on it, center drill it as best as possible and get an easy out and just back it out. You won't have to drill the hole and retap it, just drill and tap the one that is stripped. Don't waste time making more work for yourself.
By the way, those bolts are pretty stout, what the hell kind of torque are you putting on them? They are only 40ft-lbs, you might need to recalibrate your torque wrench.
If you drill and tap to the next size on the others, you are most likely going to have to ream out the foot rest flange too. Otherwise the next size bolt won't have clearance.