When I was a kid we had a doggie that hated my one Uncle. That same dog would do flips for other people but would growl at my Uncle. We could never figure out why but suspected his scent (my Uncles, not the dog) was the problem. And we also thought that the dog was totally confused because my Uncle looked a lot like my Dad and his voice was similar, thus the dog felt duped all the time..... I have not a clue as to why your dog has such bad feelings for its former Vet, but just accept it and tell him to renew his insurance. My guess is that with repeated exposure to each other, they would be fine. My Uncle didn't come around all that often and that could have been keeping the game going. By the way, the dog was a sheepdog mix named Fluffy. Fluffy was a delightful doggie and would allow me to dress her, comb and set her long hair, she would wear sunglasses and hats through the house....she was a delight for sure. And she hated my Uncle.