I have a lot of respect for vets, but you have to remember that they do not learn breed specific information. Some vets are small dog specialists, and even they aren't up with the latest news in specific breed information. The experts on breeding and what's safe, is the mother club for yorkies, and that's the Yorkshire Terrier Club of American. The YTCA is made up of experienced breeders, and I believe that they know much more about what's safe for a Yorkies than most vets. One of the reasons, I chose my vet is that he told me, he gets most of his knowledge of breed specific information from really good breeders.
Here's what the YTCA says:
The mother will probably need a cesarean, which is very expensive, and it's very possible that she'll lose the mother and/or the puppies, if she can't get her to the vet in time. Remember also, that the pregnancy weight puts an awful lot of stress on a tinies who already have fragile bones, especially the knees. Some breeders will breed tinies, but they aren't emotionally involved, so it doesn't matter to them if they die in birth. They just want to sell the puppies. Please "OVERSTEP", she might be very thankful you did, the more she learns.