I went to Naturetto to have lunch yesterday and, although they don't have many vegan hot dishes on the buffet (actually there was only one hehe), the food was good and well prepared. The salad buffet had many delicious options like mushrooms and olives. The downside is their price. I paid like 27 brazillian reais (~13 dollars) for a well served dish.
Cons: few vegan options (hot dishes), expensive