In-narratively, when a vampire feeds they are rarely, if ever, actually draining all the blood (or even half of it, by volume) out of a given (mortal) victim -- the energy/life force transfer is essentially partially physical (there needs to be substantial, though not overwhelming, amounts of blood) but equally or more so metaphysical (especially in the case of strong-blooded -- 7th or better generation/potency vampires). (Not near my books right now as most are in storage, but there were occasional references to this in different instances; the one that comes to mind for me was in Dark Ages where...