If it is just vomiting, without diarrhea, the next diagnositc step would be to do endoscopy with biopsies. There are infections that can occur in the stomach (Helicobacter for one) that will cause chronic vomiting. Also, sometimes peices of toys swallowed long ago are still rattling around in there, and can cause chronic vomiting.
If that is not possible, I might suggest putting the dog on a hypoallergenic diet such as Hill's Z/D for a while to see if that helps.
You may consider adding an H2 blocker such as Pepcid AC or Zantac. Ask your vet for doses. There is a condition called bilious vomiting syndrome that causes dogs to vomit bile when they have accessive stomach acid. Another way to help if this is the cause is to frequently feed the dog small meals or snacks, so the stomach is not empty for long. This is especially important first thing in the morning and late at night, before bed.
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The exploratory with biopsies (make sure they get samples of everything they can), is actually not as horrible as it seems. It is done routinely. The anesthesia is the major concern, but if the bloodtests are coming back fine, the risk is one I think you should take. I would still advise you have endoscopy done by a referral institution before going with an exploratory laparotomy, but if none is available, the surgery may be indicated.
If she has not been spayed, she should be as soon as possible.
I wish you luck. I know how frustrating it is to not have a diagnosis. Such is the problem with veterinary medicine. We are somewhat limited to the symptoms that animals will show, and the information from lab tests. Not always great.