First of all, I said "slightly acidic" which is exactly what your proposed vinegar solution would be. And second of all, I know that these are largely carbonate bearing rocks... that's the whole point-- to cause the outermost layers of the mineral to react away into the acidic water, thereby taking any salt that might be hiding out on these surface layers with it.
I honestly wouldn't do it myself. Not on a 500g tank. But because of the size of the tank, I understand precisely why you want to keep using these rocks and not have to buy any more.