Other things for a Vampire to drain besides blood or emotions
Fear: These Vampires must terrorise their prey rather than go in for the kill.
Alcohol: They still drink blood, but act more as filters than consumers. Other impurities in the blood can be used instead.
Youth: The life essence itself is drained. An attack by these Vampires adds a few wrinkles & a few new grey hairs. This is what keeps the Vampires immortal--they take the time away from someone else. Attacking a child causes progeria-like effects rather than growth.
Orange Juice: Okay, I'm being silly with this one, but why not have a Vampire who must feed on one specific fruit juice, but is otherwise the stereotypical Undead?
Drama: The tropes of Vampirism: black lace, gothic music, depressing poetry, et al. create the emotions upon which the Vampire must feed. They don't even really like blood, but it is sometimes necessary to drink a sip for the right atmosphere.
Insulin: A variation on the filtering Vampire above, the victims of this Undead suffer from diabetes-like symptoms. Slain victims die from sugar-shock rather than exsanguination.
Metal: Only the most famished Vampire attacks a human, and then it is the iron in the blood rather than the blood itself. Unfortunately for these creatures, they are not able to bite off chunks of steel (fangs only punch holes) and constantly are searching for some way to get the rich metally goodness they need. Metal shavings/iron filings could form the bulk of their diet, but there is a risk (albeit reduced) of sharp filings causing internal damage. Rust is easy to chew & digest, but as it is already--definitively--oxidised, they get almost no nutritional value from it.
