Oppen said:
Slags didn't prey on Modoc's livestock, they just made a farm... Arefu's "vampires" were actually more like cannibals in withdraw. Think about the blood like nicotine chewgum. At least that's what I remember from the dialogues, I may be wrong.
It's pretty much that. The Family use blood to sate their "cannibalistic" urges as a result of some sort of mutation, with Vance teaching them self-discipline and the lifestyle of "vampires" of legend in order to keep them from feeding on humans.
The Slags had pretty much made up the "Ghost" story in a similar vein (making themselves look like supernatural creatures), although it was more for outsiders rather than themselves.