My sympathies. I too have had the dubious pleasure of removing corroded alkaline (AAs) from both Maglite and Streamlight flashlights.

The cells swell up and bond/weld to the metal. That makes them very challenging to remove. After many experiments, I ended up using a cordless drill to drive a long deck screw (wood screw) through the battery. Or maybe it was two screws. Anyway, that gave me a solid anchor to grab hold of with big Vise-Grips. Next time, I would be tempted to grab the screw ends in a bench vise and pull on the flashlight body, which is easier.

I tried the vinegar thing after (strong 7% pickling variety) but it did essentially nothing. I ended up using a variety of steel scrapers to physically chip the crud out.

Good luck.