Finally got the plug out, and the new one in (part no. GM-90501549/ which is the bigger of the two for a VS 111 97'). Actually spent 6hrs, 4 cups of coffee and a lot of perspiration to get this seemingly simple job complete. It was almost bed time, i tried the vice grips once more after multiple options (grinder, hammer/chisel etc.) and i finally got the plug a 1/4 turn, the rest was easy! Unbelievable feeling, just shows no matter how small the 'upgrade', it can still give a satisfying feeling. I guess this is why we do, what we do!