First, background on magic, in my system. Every living human being has magic in them, just by being alive. Some have much magic, increased by study and practice or by being in proximity to magic (think of it as second-hand smoke), some have only a little.
Back in the mists of time (Bronze Age?) magicians developed a way to join their powers and channel them towards some great task they couldn't accomplish singly. The joining was accomplished by a blood-sharing ceremony and had a tendency to kill or drive insane one of the participants--the weak link in the chain, it was assumed.
The magicians came up with what seemed like a good idea. They would include in the joining someone (a slave or prisoner) who was dying, in hopes that the magic would be directed to that 'weak link' and spare the magicians.
Sometimes it worked as expected. Sometimes it charged the dying person with enough magic that he didn't quite die. Instead he became a sort of zombie, a mindless slave blood-bound to the magicians, and could be maintained semi-alive by small feedings of blood, the same way animal familiars were bound to a magician. And who doesn't want a mindless slave, particularly one who could be used as a sort of magical storage-jar? While it might be filled with magic, it couldn't direct or control that magic, because of not being properly alive.
But if the zombie-slave was overfed, or re-used too many times in the joining ceremony without being burnt out by it, it might become sentient. If the magician who was feeding it died or lost control of it, it might become autonomous. And would still need blood to maintain itself, but would have to go out and get that blood. Thus, vampires.
It's the magic in the blood that maintains them--although they can't control magic, they are magical creatures, and have some powers (glamoury, strength, etc.) that are associated with magic. If one takes a magician's blood to the death, it is charged with enough magic to create another vampire, blood-bound to it. (There is not a strict one-to-one exchange, by the way). For this reason, second or third generation vampires are forbidden to take magician's blood, lest they become powerful and autonomous. Or, if they take magician's blood in small quantities, they might be blood-bound to the magician instead of to the vampire master who made them.
Somewhere between the Bronze Age and the Early Modern / Late Medieval period, there was enough disruption of the magical tradition that this (fairly closely guarded) knowledge was lost to the magicians. The oldest vampires knew it, but guarded it even more closely.
Now I believe I'll go to bed, so I can be up early and work on the Dread Synopsis.