All of this talk about only Vegetarians being able to cook vegetarian meals is out of date. When Showtime and the correlating patch were released, it changed to where any Sim can make vegetarian meals. New meals also became available at the book store: Vegetarian Fish and Chips, Stuffed Tofurkey, and I don't know what else. Just go to the book store and look for the new recipes. They will be right next to the original version of the recipe.
Edit: There's another thread about this same subject. I'm going to merge the two thread together.