What was vizzini's fatal flaw princess bride?

Vizzini's fatal flaw in "The Princess Bride" is his overwhelming arrogance and overconfidence in his own intelligence. He consistently underestimates his opponents, particularly Westley, believing he can outsmart anyone. This leads to his demise, as he fails to recognize the true nature of the game he is playing and ultimately makes a critical miscalculation that costs him his life.