Have your dog tested for low thyroid. My lab had the same thing for a couple years and none of the vets could figure it out. Then she got really over weight (for no reason, we were not overfeeding her) and she got a "rat's tail" and dandruff looking skin.
Finally a 3rd vet did a thyroid test on her, and after 2 months on the thyroid medication she is a new dog now and hasn't vomitted once, her hair grew in on her tail, and she lost 10 pounds in 2 months without being on a diet - it was all the thyroid.
Why did the vets miss this? You must ask them to do the test. Low thyroid is VERY common with labs and it usually onsets around 3-4 years old. Vomitting clear like that is JUST what my dog did too.