I have been two times at El Vegetariano and I really appreciated. I took the menu (S/.11, but 12 on Saturday) with starter, main course, dessert, drink and yogurt. The service is quick, the food good and the portions aren't small. It is also possible to ask for a 'doggy-bag' for the yogurt, dessert and drink if you prefer to.
I like the atmosphere and the quality of food, even if it is not really refined o surprising. I've heard that the founders of El Vegetariano used to work at Bircher Benner before, so basically you get the same food but cheaper. In fact, it is possible to buy Bircher Benner products as well in the restaurant.