It's hard for a vet to be up on the requirements of each breed. I'm sure your vet is very capable, but I think that an expert breeder's opinion on this matter is the one that should count; after all, it's the really good breeders that train the vets. Here's a link that explains the 12 week rule, it's about Maltese dogs, another small breed, but applies to yorkies as well.
Why do Maltese puppies need to be 12 weeks old before going to a new home??