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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