I have been thinking about this overnight but I am not a chemist so I am unsure of the reactions. I do know that the penny gets shiny when you remove oxygen and impurities like sulfur from the surface. So lets reason together on this....The salt breaks down into hydrogen and chlorine in the water and produces a slightly acidic, HCl, solution. This breaks down the copper oxide pretty well. I do know the chlorine works well to clean of the copper oxide, I use "comet cleaner" with chlorine to clean brass. Now the vinegar has an acetic acid, but this is carbon, hydrogen and oxygen and I think that the salt combines with this to form a slightly basic solution and generates CO2. I would be interested in seeing if the vinegar salt solution with water added would improve the cleaning capability.