People seriously blow this way all out of proportion. There is no big difference. Most Mbuna do not eat very much protein in nature. Some Mbuna are algae scrapers, but the non algae scrapers do not need different food.
By "carnivore", they mean that they eat little tiny creatures found in the algae, in cracks in the rocks or sand, or floating in water... as well as some algae.
By "herbivore", they mean they scrape or pick strands of algae and also eat the tiny creatures in the algae, as well as eating the tiny creatures floating in the water if they are available.
Melanochromis chipokae and a few other Melanochromis are the only ones actually sort of predatory, but they eat anything they find as well.
You can put some romaine lettuce or zucchini in the tank sometimes. There are no actual vegetarian fish foods, just avoid high protein foods. Mbuna will eat them all. Avoid too much food with protein since Mbuna can get really fat and overgrown.