I read a post the other day at the always-fascinating rogueclassicism blog that should be of interest to anyone gaming in a Weird Roman setting, or looking for a new twist on vampirism. No sparkling, hair gel, or abstinence, I promise. I recommend reading the full original article, but here's what I thought was most interesting:
Here's Pliny the Elder (trans. John Bostock)
Epileptic patients are in the habit of drinking the blood even of gladiators, draughts teeming with life, as it were; a thing that, when we see it done by the wild beasts even, upon the same arena, inspires us with horror at the spectacle! And yet these persons, forsooth, consider it a most effectual cure for their disease, to quaff the warm, breathing, blood from man himself, and, as they apply their mouth to the wound, to draw forth his very life; and this, though it is regarded as an act of impiety to apply the human lips to the wound even of a wild beast! Others there are, again, who make the marrow of the leg-bones, and the brains of infants, the objects of their research!
And Celsus:
Some have freed themselves from such a disease by drinking the hot blood from the cut throat of a gladiator: a miserable aid made tolerable by a malady still most miserable …
The post's author goes on to hint playfully at a connection between Julius Caesar's interest in gladiatorial games and his purported epilepsy, but any GM running a horror-tinged campaign can surely see the potential in this. Here, we have a form of vampirism still tied to superstition, but without any overt supernatural elements. It's one thing to locate and destroy vampires when you're dealing with immortal undead monsters with relatively well-known powers and weaknesses, but quite another when the monster you're looking for is an ordinary person who casts a reflection, can walk freely in the sunlight, has no fear of holy symbols, etc.
Your range of potential "vampires" includes everyone from family members pushed to extremes to find a cure for a stricken child to the sorts of unscrupulous persons hinted at in the end of the Pliny passage -- physicians and sorcerers conducting vile experiments in secret. The efficacy of any of these cures is up to the GM, of course. Rumor and superstition are powerful motivators all by themselves, and an unsuccessful "vampire" might simply be convinced that there's a flaw in their methods, or that they haven't found just the right victim.