Vera's identity? (spoilers)
I'm wondering if Vera was the child that Hannah babysat that night. Hannah might have offered her to the devil and that effected Vera/made her evil in some way. Remember that Vera says her parents died (maybe she killed them?) and there was emphasis that Vera didn't want the company of people nor did she ever want children.
Mostly this would give some sense to the ending and Vera's decision to take the devil child, because this would mean she has some bond with her.