Your cat is a true carnivore with unique nutritional needs. He can't obtain a significant amount of required nutrients, such as taurine, protein, fatty acids, Vitamin C and niacin, from vegetables.
Be wary of vegetarian cat food diets that claim to be healthy. Even if a cat seems to be doing fine on such a diet, there is no way of knowing what the long-term health effects are. Even if you are a vegetarian, don't force your cat to live on what for him is an unhealthy diet. Always choose nutritionally balanced cat food that contains meat.
For more information, please read the story Feeding Your Adult Cat