Character Analysis
Vizzini is basically a smart aleck. He's really proud of his brains in the same way that Inigo is proud of his sword or Fezzik is proud of his strength. As he tells us directly:
"I, Vizzini the Sicilian am, speaking with pure candor and modesty, the slickest, sleekest, sliest and wiliest fellow who has yet to come down the pike." (5.844)
But like Inigo and Fezzik, Vizzini is destined to be beaten at his own game by Westley (a.k.a. the man in black). Unlike Inigo and Fezzik, though, he isn't destined to then join forces with Westley. Nope, his hubris gets him killed.
In a battle of wits, Vizzini is too proud to realize that he's been tricked by Westley into drinking poison. The guy's pride is so great that he doesn't even realize he's lost until he has dropped dead, boasting of his victory in the intellectual dual until, well, he just can't boast any longer. Or, as the narrator says:
He was quite cheery until the iocane powder took effect. (5.901)
But whatever. It totally serves the guy right. After all, he accepted money from Prince Humperdinck to kill Princess Buttercup. Maybe in his next life he'll concentrate less on his ego and more on his kindness.