Your vet was out of line. Period.
A
professional would have told you (his owner) that that was unacceptable behavior and to get your dog under control. The vet could have refused to treat, examine, etc, your dog until you had him under control, or he could have asked you to leave. He did not have to hit him. Because it is your job and responsibility to teach your dog manners, not the vets.
I would get another vet. Doesn't matter whether the tap was a light tap or not, he should not have tried to discipline unless he was in fear for him self, which doesn't seem to be the case.
bottom line, it is your choice where you take your furbaby for care. But it is also your responsibility to train, socialize and raise him to be a good canine citizen
