"
Just heat the heck out of it and then try the vice grips again. Get it
as hot as possible and aim the flame right at the bold.



Would there be increased risk of shearing the bolt off
after heating (assuming it still won't yield at the threads)?


No. Heating is pretty reliable as long as you don't burn anything around
it.




Or you could put dry ice on the bolt. Put continuous torsion on it while
it is cooling and it might just fall out when it shrinks.


I used to run into this all the time in my former business. Heat the bolt
till it's cherry red and then splash cold water on it.
The bolt will expand with the heat and contract with the cold. It'll come
out like a baby.