I really enjoyed eating at Refeitorio OrgÃ¡nico during my stay in Rio. I was there about five times and loved it every time.
The buffet livre (all you can eat buffet) is 23R$ (december 2012) and offers a huge variety of different dishes.
Usually there are two soups, about 10 to 15 different salads plus the same amount of main dishes and a heated plate with piles of tofu, seitan and grilled vegetables.
Additionally you can choose from several very tasty vegan sushi/maki rolls, including sweet ones with banana and cinnamon.
The food is from all around the world: I've seen sushi and miso soup, hoummus and arabic salads, pasta, yakisoba and traditional Brazilian dishes like a great vegan feijoada.
Going there more often you'll realize that a part of the main dishes, salads and makis are the same every day, but even after eating five times there I did not get bored as there is so much to choose from.