You probably can't grab it with Vice Grips or you would have tried that.
If the JBWeld doesn't work, you might try using a Dremel with a cut-off wheel to slot the broken end and then try a screwdriver. I've had good luck with that technique. Depends on if any of the bolt is exposed.
Since it wasn't torqued down, another technique is to try to catch the broken lip with a sharp punch and then unscrew it by gently tapping the punch counterclockwise until enough is exposed to grab it with Vice Grips.
If you resort to drilling or using the Dremel, be sure to mask off the head to keep grit and metal shavings from getting into it.
Good luck,
Matt
