There's not that many a la carte restaurants with a big selection of veggie food, the standard seems to be one or two appetisers, and one entree often involving pasta.
Saying that Boma has a fantastic selection of veggie food on the buffet, and it's really good.
Jiko has a good selection of vegetarian appetisers, and a main course involving tofu which was good.
Coral reef had a delicious vegetarian risotto when we were last there and the vegetarian lasagne at Chefs which is pictured above was very good too.
California Grill always has some tasty vegetarian options, the flatbreads are lovely.
Flying fish have no vegetarian dish on the menu, but the chef made a sampler platter of the vegetarian side dishes which was good.
Overall we spent 3 weeks in the world last year and were almost always able to find a decent vegetarian option at our meals.