Fridge Brilliance: "Vanity" is another way of saying "pride". Of course that would be the Devil's favorite sin, being the reason he Fell and all.
Fridge Horror: When Satan gloats to Kevin about having sex with Mary-Anne, he says that "on a scale of one to ten, ten being the most depraved act of sexual theatre know to man, one being your average Friday night run-through at the Lomaxes' household, I'd say, not to be immodest, Mary Ann and I got it on at about... seven." This after Milton raped her, left her with gashes all over her body, and left her so traumatized that she later committed suicide. Now imagine what a ten out of ten must entail.