Alright-- I stand corrected: I'm used to cleaners that dissolve the residue I'm trying to remove, put it into solution.. Vinegar doesn't do that, apparently; it just loosens dirt's grip on the surface to be cleaned. I shook it around a few times, taking on a pungent odor myself, and the dirt visible simply came off the tank. A clumsy go-around with a parts cleaning brush completed the task, to the extent possible given the complicated interior of this tank. Pic to follow.
Weird stuff, vinegar. You learn something every day, if you're lucky.