"The long soak in acidic solution dezincified the surface of the brass, leaving red copper. It should be a surface effect that will polish off."
Correct. Vinegar is a very mild acid, we eat the stuff all the time in salad dressings, pickles, etc so it's not going to damage cartridge brass even after a full strength over-night soak. A half hour or so is usually enough to turn the tarnish pinkish tho; diluting simply means it will have to soak longer. Then flush it well in running water, dry it, tumble it and you won't know it from non-tarnished cases.