[ Karkat isn't honestly sure if it's okay to go around and just tell other peoples' stories...

ah what the hell. Karkat is totally up for things like that because he's into observing all that he can and then extrapolating it. ]

It's because originally Vanitas came to be after Ventus' heart was shattered. Vanitas is the darkness that once resided there. So yeah, he's kind of human, since Ventus is. But he's also sort of not.