But that's Sora. He was always duped too easily because, as Ansem the Wise put it, he sees the good in people. I realize he wouldn't be a good Master because he never had any encounters with Darkness, but he obviously has a part of Vanitas in his heart since he has Ven. I just think that it was a bit harsh on Sora. Granted, they almost lost the most powerful Keyblade Wielder before this game, so I'd imagine Yen Sid was a bit pissed.