How did he cheat? He asked, "Where is the poison?" If he said, "Which goblet contains the poison?" then you might have a point, but asking, "Where is the poison?" leaves open the possibility that it's in both goblets. Vizzini's error was failing to consider the possibility that Westley was immune. There was no cheat. Westly simply outsmarted him. Actually, he defeated him the moment Vizzini agreed to the challenge, which was a stupid move because it removed Vizzini's only advantage (holding the knife to Buttercup's throat).
Agreed. The battle of wits was never about figuring out WHICH goblet had the poison in it. In fact, if Westley had only put the poison in one goblet and Vizzini had to pick, that's not a battle of wits at all. That is a guessing game.
The battle of wits involved the entire encounter. When Vizzi agreed to not only put down the knife but then to also let Westley set all the rules of the game, he lost. He gave up his advantage because truthfully, as long as he held the knife to Buttercup's throat, they were not in a stalemate. Westley outsmarted Vizzi by having him agree to the battle and that is how Westley won. He definitely didn't cheat.