You could try a hypoallergenic diet. Look for "limited ingredients" cat food that doesn't have grain in it, which is the cause of many feline allergies.

Also, chlorpheniramine has been proven safe in cats. It comes in 4 mg tablets for humans - cut one in quarters with a knife, then grind one piece up with water and mix it into a little bit of particularly tempting canned food, and see if that helps until you can pinpoint the allergen. Make sure you get JUST chlorpheniramine, not with anything else mixed in (it's usually just called "allergy relief tablets" in the drugstore).

And I agree that you probably need a new vet.

Love, Columbine