A good, unpretentious cafe, one street back from the main street up to Hallgrímskirkja (but still very easy to find); the only nod to any 'right-on' vegetarian stereotypes would be the photos and posters of Tibetan Buddhism ;)
The menu couldn't be more straightforward, either: a soup of the day and/or one dish of the day. And that's it. One could have a half portion of the main dish, a full portion, the soup, or both. Don't fancy either? Come back tomorrow (the schedule's posted for the coming week).
A half-portion of that day's vegetable lasagne, served with salad, was tasty and more than adequate for lunch; the soup looked good too. I might recommend the full portions for a main meal, but I ought to also qualify that: at least on the day we visited (a Friday), the cafe closed at about 18:30.