Was he vomiting at the time the blood work was done, or is the vomiting a new development? Are there any other symptoms?
One possibility that comes to mind is inflammatory bowel disease. Blood work wouldn't show anything if this is the problem. It can cause intermittent vomiting and/or diarrhea. You would need to discuss this with your veterinarian. Treatment often involves corticosteroids and diet change. If you wanted to try a diet change on your own that would be fine. You want to pick a food with a protein source that your kitty has never had. IVD (Innovative Veterinary Diets) makes some diets with unique protein sources such as duck and rabbit that might be a good choice. You can also try adding a supplement to your kitty's diet. Digestive enzymes often help pets with chronic vomiting problems. Prozyme is a wonderful one that I've personally witnessed helping several pets. It can be bought at most pet stores or on-line.
Here is a link to a page about inflammatory bowel disease:
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