First thing to do is remove the front trigger guard screw if you haven't already done so. The trigger guard is virtually pivoted at the back to a clamping action toward the front. The sharp tap in what's left of the slot with a correct width screwdriver may be all it takes. If you go for the vice grips, a layer of electrical tape over the part of the jaw touching the triggerguard may save you from marking the bluing.

Failing these, with nothing left to lose before drilling, try tapping your smallest punch at the end of the slot to rotate the head. It will either turn or shear off- which will make it easier to centre a centre pop mark for drilling anyway.

Good luck