Hi,
this sounds abit like trauma aggression. It's not that your cat smells of the vet but that when he came back he may have been scared about his visit and has been reacting differently to your ther cat. This change in behavior is what is causing your other cat to react aggressivily towards them. Usually the behavior will right itself after a while s they both calm down and get used to each other again. If the behavior continues you will have to go back to introducing them again as if they had never met. See my page on the cat training pages about introducing cats.
