At the quieter north end of Playa Zicatela (not so close to the noisy beach bars) I found El Cafetal; a very pleasant spot and a very nice surprise to see it had a fully vegetarian menu when I visited (although it didn't really promote itself as a vegetarian place so maybe this changes?).
The menu was quite simple and restricted to breakfast and lunch but the coffee was the best I had in Puerto Escondido, and the food was very good as well. And served up in a friendly pleasant chill and quiet atmosphere.
Cons: Limited menu, Limited hours