He is so cute!!! Looks yorkie to me. Just because he's bigger doesn't mean he's a mix. In his pedigree (family tree) does it state the weights of his relatives? I'm sorry to say but your vet may be wrong. His statement on weights is more an opinion than it is fact. If it's in your dog's genetic makeup to be a bigger dog, he will be a bigger dog no matter what size his parents are. He can be a throwback from generations past where a larger yorkie was used in the mix. Nothing wrong with that, unless you expected to show him. For us pet owners it just means more to love.
If I was in your shoes, I would get him DNA tested so I would have a bigger case against his breeder. Who gives a cat vaccine to a dog?!?! She doesn't sound very bright.
Did the vet tell you if the cat vaccines were dangerous?