'Healthy' catfood is making my 13 yr. old cat sick?

At 13 years old, my cat has fought urinary tract crystals and won! However, he now has to eat a special food that I buy directly from the vet. It's expensive, but I don't care about that I'd pay anything in the world for this guy. What I am worried about is the fact that he gets sick and vomits after every meal. I've talked to the vet and the cat has been in for a check up. Nothing seems to be wrong health wise, so we narrowed it down to the food causing the problem. There are other brand names (Iams, Purina (sp) One) that make a food just for Urinary Tract problems. Should I try these other brands to see if he stops getting sick? Or should I stick with the vet's food and hope for the best?