Vivi is simply and artifically created intelligence, like Tidus, that was created with the gag of the "unknown" face as the face in old Final Fantasys are covered when wearing the hat. In Vivi's case, a mist creation, it is more like the Black Mages being an actual species. Their eyes, the yellow light, even fade away when they die. Well, somehow that is fitting because you talk about the light in your eyes that vanishes.