If you're looking for the one of the cheapest restaurants for vegetarian dishes, you have to consider "Oriente no Porto"! The location is good - although it's located in typical small street that's a bit hard to find, it's only 5-7min on foot from S. Bento Station.
The people are nice and the meal was fine! The meal the day I ate there had some Indian influences: it was some kind of meatballs with rice and a spicy side dish that I don't know what it was. The soup was ok, they also gave us (caramel?!) tea and a dessert ("alfarroba" and chocolate mousse). We only paid 5EUR (soup+main dish+tea+dessert), which is pretty good in Oporto!
The restaurant itself seemed ok. I think they have yoga classes for what I read in the other comments, but for what I saw there, there are also workshops you can participate in.Pros: Price - very cheap indeed, Food is nice