By 'evaluating' I meant to help you determine how they measure up to the breed standard. Your vet will only be able to tell you if they are physically sound, but not if they are quality representations of the breed that *should* be bred. (Generally speaking, your dog may be a great pet, but may not possess qualities that should be passed on to future generations. Unless your vet is a long time Yorkie expert, or professional judge, he/she will not be able to advise you on that.)
Something to consider: You state that your vet says they "are perfect for it"... How on Earth could ANYONE, vet or otherwise be able to determine that when they are only 12 weeks & 6 months of age!?
They still have a lot of growing up to do before that can be determined.
Ellie Mae gave you some good advice on how to go about finding a mentor. Attend shows, introduce yourself, contact breeders in your area, etc...keep trying and trying until you find someone. What about the breeder you got your little ones from? He/she would be a good place to start, and, they should know their own lines pretty well and could help you out.