I had the pleasure of staying at Samasati recently during a too-short stay in Limón, CR. The food here is all vegetarian, though they will cook fish by request for an extra charge, so it is not a totally vegetarian kitchen if that bothers you. Everything is served buffet style (unless there are too few people staying there to justify preparing a buffet line).
In general, being vegan in Costa Rica is pretty easy, but Samasati made it even more so. Every breakfast guaranteed the freshest fruit you could imagine and black beans and rice. They occasionally served sweet plantains as well. They also always had coffee and fresh juices.
Lunches and dinners offered many vegan options. Vegetarian casado, a sort of veggie fried rice with beans, was frequently served. Samasati also frequently served sauteed vegetable dishes like sweet potatoes, eggplant, and squash. Nearly every meal during my stay offered a variety of soup, though not all were vegan. Fresh green salad was also almost always present at lunch and dinner.
There were a number of things that Samasati did well, particularly traditional dishes (and/or accompanying vegetarian modifications). A couple of soups and sauteed vegetable dishes surprised me by how good they were. Some of their tries at other cuisines was not as impressive, like the Asian noodles that were pretty much soy sauce, soy crumbles, and pasta.
My only complaint about the meals here is that they didn't indicate which dishes were vegan or otherwise contained allergens. I found this just a tad surprising as given the general aura set by the center. Of course, it never hurts to ask, but if you are vacationing and have limited command of Spanish, you may encounter a language barrier with some of the cooks.
And, of course, all of this is irrelevant unless you are a guest at the Retreat Center; I'll save most of that for another review, but will say that I had a pleasant, relaxing time at Samasati.