I am confused as to why your vet is not suggesting further testing being done by a specialist as they can not figure out what is wrong. Even if your dog is gaining weight throwing up all the time is not normal as it putting a huge strain on your dog. From a couple of medical websites.
"vomiting can lead to swelling of salivary glands, electrolyte and mineral disturbances, and enamel erosion in teeth."Since Yorkies are prone to dental issues by not addressing this problem you may have serious dental issues down the road.
another site "it does damage the esophagus because the stomach acid isn't meant to be in the esophagus."
from a vet sites
Vomiting Causes of Vomiting - Why Your Dog is Vomiting
For specialists
Veterinary Specialty Care - Medical Care Services Veterinary Internal Medicine Practice of Northern Virginia Internal Medicine | The Hope Center For Advanced Veterinary Medicine
I am so sorry your baby and you are going through this and I hope you find answers. Sending prayers