I think that what is happening, is that the salt adds sodium ions to the solution, thereby forcing the equlibrium in dissosiation of acetic acid to acetate and H+ ions further to the right.
It happens because Na gives it's electrons easier away to water than Cu does, and because acetic acid is a weak acid, normally giving only 1 of 100 H+ ions / acetate ions per 100 molecules of CH3COOH.
An exess of salt will keep the copper clean, even with no acid, and force Cu to first release the green Cu2+ from it's surface, and next for solid copper itself to oxidice, slowly.
So You can clean copper by just adding salt and water, even though the process is slow.I have tried this with copper nails. In a soultion of of water and salt, where salt remains solid at the bottom, the nails are still shiny orange, and the solution has become almost emerald green, after a couple of months.
I am not sure of the reaction mechanism here, but I have become more convinced that what I claim above really is happening.
Try doing this: Add liquid soap to two glasses, and pour in the same amount of water. Stir the solution. Add the same amount of vinegar as water and soap. Watch what happens. Then add salt (sodium chloride), but just to one of the glasses. Stir and wait for 15 minutes. Observe the very different results.
"You are the salt of the Earth ..."