As Scotty said, it's probably stripped. It's only aluminum after all. Instead of using vise grips, see if you can get a flat head screwdriver under the bracket at the bolt and pry upward while trying to unbolt it.

It will be damn near impossible to get a drill and tap in there. Your best bet would be to get the bolt out, and saw off the aluminum tube that the bolt goes through and find a nut and washer that will fit.