Well, as **AClockworkMelon **and **Chronos **already pointed out, Vizzini apparently did not believe that the MiB was willing to bet his life on those odds. He expected that the MiB would try to cheat. That’s why he switched the goblets when the MiB’s back was turned rather than openly choosing the cup he believed was free of poison.
In Vizzini’s mind, the MiB’s plan was probably something like this: If Vizzini chose the poisoned cup then great, he’d be dead, and the MiB could grab Buttercup and go. If Vizzini chose the poison-free cup, the MiB could grab the knife from the table and stab him, then grab Buttercup and go. This is almost correct, although wrong enough to be fatal for Vizzini. But it was true that the MiB had no intention of drinking anything that would kill him, and that the MiB planned for Vizzini to die regardless of which cup he chose.
I don’t think there’s much of a tell when the MiB says “you’re just stalling now”. But when Vizzini says “you could have trusted your strength to save you” I think he gives just a bit real tell because Vizzini is hitting awfully close to home with that. Having said that, I also think that he’s trying to give a false tell there as well.
I just saw the scene on YouTube, and, not having seen the movie nor read the book, my impression would be that Vizzini is not very smart. His reasoning is really dumb. For him to be smart, I would need to assume he really was stalling. But then using that dumb of a tactic to get him to look away makes me think he wasn’t.
The reasoning is not the point. He’s throwing bullshit out there to see what the MiB will react to. Vizzini is counting on his superior intellect and observation to figure out when the MiB gives his game away through body language. It doesn’t matter what he says, really. He could have said, “There is a full moon in a week’s time, so that means you must have put the goblet in front of you. But the solstice was last Tuesday, so clearly that means you put the goblet in front of me,” and it would have played out exactly the same. He was looking to see on what conclusion the MiB would blink.
FWIW, for years I interpreted the scene as showing that Vizzini was a foolish blowhard who used “logic” that was only slightly more sound than the witch scene in Holy Grail and switched the cups because he thought this was a genuinely brilliant trick. Later I decided that Vizzini was stalling while he tried to read the MiB’s poker face, then switched the cups as a deliberately childish trick to show contempt for the MiB’s intelligence. What I think now is described in my previous posts.
I feel the “battle of wits” scene is pretty entertaining regardless of which of these interpretations you go with, but I think it both makes more sense and is more dramatically satisfying if Vizzini is genuinely intelligent. At this point the MiB has already beaten two worthy opponents at their own games, and it would be sort of underwhelming if defeating Vizzini required only outwitting an idiot.
Tangentially, there’s a subtle gag related to this scene that I also didn’t get for years. Prince Humperdink correctly identifies the poison that killed Vizzini as iocaine powder after sniffing the empty vial. But the MiB said iocaine powder is odorless. So how did the prince know what it was? Well, Vizzini had apparently dropped dead suddenly, and there was a vial lying nearby that didn’t have a suspicious smell!
