In Rio I recommend the restaurant "Vegan vegan".
Rua Voluntários da Pátria, 402 - Botafogo (21) 2286-7078
There are some vegetarian restaurants too(like "Bardana's" at Centro) but you have to pay attention if that food you want really is vegan.
On Saturday mornings you can buy some organic fruits in the organic fair at "Praça do Russel-Glória".
Good luck!
