If they autograph, they will autograph anything within reason. Not all characters autograph, but the "major" ones (fab five, princesses) definitely do. The only limits I've heard are not signing on people, and not signing on something a person is wearing.
I would take along an autograph book as a back-up. As others have said, the characters will autograph the large blank vinylmation (I’ve seen guests with a signed one awhile ago, when vinylmation was really popular), but some have reported that some of the “head” characters cannot because they don’t have the hand-eye coordination necessary to sign something that contoured (basically their costumed hands make it to hard to hold the vinylmation and sign it at the same time).