This is what I call a lucky brake. Lots of bolt to hang onto.
I , most of the time only have three threads to try this with.
Heat the Aluminum casting with a Plummer's torch well, Then Tap the end of the bolt a few times to try and brake the bite.
Hopefully, then you can use the vise-grip and it will just turn out. If not, do steps one and two again. Good Luck.