This may be one of Rio's best kept secrets. The entrance is completely unmarked; it's only after you go upstairs that you hear, smell, and see the restaurant.
I went on a Wednesday based on a previous comment, and the selection was all vegan. I can imagine it wouldn't be as appealing on other days, but it was pretty tasty, with a nice (though pretty simple) selection. The dairy-free deserts were great too.
It's clear that this place is very popular as it was nearly packed when I was in. The staff was very friendly though – if not a bit over-efficient, removing my plate with a bit left on it when I'd gotten up to get another juice.
For a vegan, it doesn't have the same variety and level of skill as a place used to working with v plates all the time, but still a great spot to visit in downtown RJ.
Cons: limited selection, limited hours, only vegan on Weds