Re: Cant get a screw out
I agree that heat is not a good idea. Not only because of potential damage the o-ring on the plug and nearby seals, but heat can easily loosen or blister the paint and cause the development of corrosion under the paint as well.
The reason I suggested a ground down drill bit above (assuming your plug is a slot rather than Phillips, is that you need a tight and wide fit to avoid further damage to the slot. Slot damage is usually caused by screwdrivers not wide enough to do the job properly. Using a flat blade drill bit and grinding it to tightly fit a stubborn vent plug will provide you with the best chance of rotating that plug loose without further damage.
If you do not want to make such a tool, the impact driver would be the next best choice, but because the blade is a loose fit and most likely is smaller than the width of the slot, there is still significant risk in deforming the plug further.
Whatever you do, good luck. But remember, you only get one good chance. Once you fatally damage the slot, you have to drill it out and reverse tap it. That brings up a host of new potential problems, ie. filings.
