get a better pair of vice grips.. if there is still a head on the nut, regardless if its rounded, jam some vice grips on there but do them up soo tight that its almost hard to get them to clamp, after this your better of knocking the vice clamps with a hammer (leftie loosie rightie tightie), than trying to undo them with your hands as the impact will have a better effect..
if you can't undo them with this, which i find hard to believe then get someone to weld a bar on there, if your doing it right it'll either come out or your'll snap the head off the bolt, if this happens i wouldn't bother with an easy out as its well and properly stuck all you'll end up doing is snapping the easy then its just a bigger pain in ass..
next step work out what size its meant to be and drill and re-tap the thread..
good luck