Vayne's intentions are never made totally clear, so it didn't appear to me that he "suddenly becomes a cartoon bad guy". For all we know Vayne's speech in Rabanastre near the beginning was merely a ruse, and his vicious, scheming side is shown from early on anyway. Certainly I never get the impression that his plan was simply to free humanity from the Occuria for the sole benefit of humanity. He may have wanted that, while at the same time taking advantage of the situation to maximize his own power. Or perhaps was willing to go to great lengths to achieve this "liberation", believing that any ends justify the means. This is hardly rare in real human history - there was much talk during the cold war of "saving western christian civilization/democracy/freedom from communism", while the reality for many people on the recieving end of this "salvation" was death squads, torture, disappearance and carpet bombing (no, I'm not interested in discussing politics here). The whole plot is quite open ended and morally ambiguous, perhaps excessively so for a video game.