Update: tried vinegar on a rusty shutoff valve in my Kansas City basement and it actually turned
Had a gate valve under the sink that was frozen solid, so I soaked a rag in plain white vinegar, wrapped it around the stem, and let it sit 3 hours while I did other work. It broke loose with maybe half the force I'd been using, which saved me from cutting pipe and sweating a new valve on a Tuesday night. Is vinegar soak a normal first move for you folks, or do you go straight to penetrating oil?
Did you try heat first or go right to the vinegar? I've had good luck hitting a stuck stem with a hair dryer for a few minutes, then hitting it with a shot of PB Blaster while it's still warm. The heat makes the metal expand a hair and lets the oil wick down into the threads way better than cold.