Salt? Never heard of that. If you want a dog to throw up, give him a tablespoon of hydrogen peroxide and wait 10-15. If he hasn't thrown up by then, give him another tablespoon. Basically the same thing as the salt, but with hydrogen peroxide instead.
It will definitely make him throw up if that's what your vet thinks is the best thing for eating a diaper. If it is or isn't, that I don't know.