As others have mentioned, finding vegan food in the cold Ushuaia is almost impossible, specially considering that even in the market you might not find a big variety of produce to cook yourself based on the season.
After finding this place I managed to have several good meals without having to cook. The food quality is average and the lady is very nice. The price is very cheap.
If you come after hours you can ring the bell if the curtain is not yet closed. They will come and serve you.