Numan only as a very last resort would I drill out the stud, even 10mm is enough to get a hold of you may have to put a similar 6mm stud just at the edge of the other side of the vise grip it will keep the jaws parallel so they stay on the stud, failing this maybe weld a nut to what remains...heat is the key, and a rocking motion one way then the other usually works but always reversing the direction when it begins to tighten...strangely enough the first direction to find some 'give' is often the clockwise direction because the screw wont be bottomed in the hole, the amount of free movement will gradually increase...I have done this countless times myself it almost always ends in success,.................. drilling steel from ally is fraught with danger!
apologies if im telling you how to suck eggs mate
as to my birthday, well remembered it was yesterday