Casa da Sopa's seating is all under outdoor canopy. It is very spacious and lush with a lot of brickwork. I quite enjoyed the setting. The all-you-can-eat buffet costs about $10 USD per person. They had 4 vegan soups when I went: vegatable, miso with tofu, black bean, and pinto bean. I believe they also have some more soups that are vegetarian but not vegan. The food bar is minimal, containing cold items to add to your soup. Amongst the veggie offerings were seaweed, cilantro, fresh onions, dehydrated garlic, peppers, as well as some bread. For dessert there was rice pudding and mango lassi (presumably not vegan but I did not ask). Check their website for correct hours. When we went they closed at 10:30pm. The soups were all pretty salty but not overly salty, though I like a fair amount of salt. The flavors were good, although they did not blow my socks off. The place is fairly out of the way from centro, but it's a good enough option that's open late in a place like Manaus that has few vegan options.
Cons: Out of the way, It's mostly just soup