Sacrifice a creature: Vampire Aristocrat gets +2/+2 until end of turn.
If you sacrifice an attacking or blocking creature during the declare blockers step, it won’t deal combat damage. If you wait until the combat damage step, but that creature has been dealt lethal damage, it’ll be destroyed before you get a chance to sacrifice it.
You can sacrifice Vampire Aristocrat to activate its own ability, but it won’t be around to get the bonus.