Acetic acid (vinegar) and citric acid are "weak" acids, and have a relatively high pH. Neither will dissolve base metals (producing hydrogen), but they will dissolve oxides which form tarnish. Of the two, citric acid is a much better solvent because it chelates (renders soluble) metal salts. It is the primary ingredient, along with detergents, in Hornady's ultrasonic case cleaning solution.

My personal method is to do both. After sizing and depriming cases with a carbide die, I wash them in an ultrasonic cleaner to move carbon and most tarnish, including tarnish caused by heat, inside and out. Most of the primer pockets come out clean and bright too. I dry them pretty well (so medium doesn't stick), then tumble them to remove scratches and any remaining tarnish. If there's any moisture left, the tumbling medium (corn cob + cerium oxide) takes care of it.

The cleaning action of an ultrasonic tank is through cavitation. It doesn't matter if the cases are standing or laying on their sides, or even stacked up, as long as they are all under "water" and the tank is not overloaded. The Hornady tank holds about a quart of liquid and 1 to 1-1/2 pounds of cases.