Such a strange offer. Vanitas doesn't really understand why Sora insists upon it. Doesn't he realize that Vanitas is made of darkness? Even if it isn't always evil, it doesn't really have many good qualities. Vanitas is not a good person. He's not even a nice person. To top it off, Vanitas doesn't really see where he needs to change.

"I couldn't be me," Vanitas says simply. "What I am isn't what you and yours stand for."