Put the vise grips on TIGHTLY and place rags around to catch any shavings if they slip. Ideally and probably unnecessary in this case, use two pair of vice grips with the jaws directly opposing each other. The grips and the fastener form a "T" when correct, the broken bolt is the upright and the vice grips are the top/horizontal of the "T".

Apply equal torque to each vise grip, this centers the torque over the fastener and reduces the chance of twisting it off to one side.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer