Vortigern99
Sorcerer Supreme
Streiber's take is terrific fun, but IMO his is only one possible "rational" approach to the subject. Others include mutant, alien, and mutant alien. 
Also, Streiber's explanation that vampires are a "parallel species" ultimately falls short of describing the origin of their deathlessness. Immortality is not a natural feature, but must be explained by some supernatural or super-scientific (IWO "sci-fi/fantasy") means. Irrational conjecture and invention go hand-in-hand with vampire lore.
Pure rationality spoils the fun of vampires, which is why I suspect Streiber held back from touching on his character's "undead" quality from a purely biological standpoint.
Also, Streiber's explanation that vampires are a "parallel species" ultimately falls short of describing the origin of their deathlessness. Immortality is not a natural feature, but must be explained by some supernatural or super-scientific (IWO "sci-fi/fantasy") means. Irrational conjecture and invention go hand-in-hand with vampire lore.
Pure rationality spoils the fun of vampires, which is why I suspect Streiber held back from touching on his character's "undead" quality from a purely biological standpoint.
I was going to say some sort of rational explanation for the phenomenon as well, but Whitley Strieber already did that in The Hunger (made a decent movie, too...).
The vampires are an example of parallel evolution; they are not human. They mimic humans and live among us, and can to some degree "turn" humans who can gain long life thereby. The humans are still mortal, however.