Buffy Mythos
From my actual reserach ...
The myths about vampires were most prevalent around the turn of the eighteenth century. At this time witches were still being burned at the stake (Wickwar 134). Anyone could become a vampire, because a vampire was just a dead person who rose from the grave to feed on the blood of the living (Britannica 868). Any corpse qualified. The word vampire is of Slavic origin, and means "bloodsucking ghost (Wickwar 144)." In other lands vampires were known by other names, and although each creature was somewhat different, they all had features alike. The Hebrews had the lilith, the Romanians had the lamis, and the Irish and Arabs had comparable ghosts as well (Hill 3923).
The Romanians also had words for people who would become vampires after they died. In Romania, it was believed that a person born with a caul (amniotic membrane still attached to the head and forming a veil), a small tail, or with hair covering his or her body was destined to become a vampire after death. Such people were called strigoi vii [singular: strigoi viu], which means 'living vampires.' A dead (or as we say 'undead') vampire was called strigoi mort. Strigoi vii had the ability to voluntarily leave its body at night (Strigoi 1).
You may wonder about other ways that humans may become vampires.
At one time it was a generally accepted possibility that any dead body having become bewitched would become a vampire; or, for that matter, anyone who had been cursed by their parents, or had been excommunicated by the Church was thought to be qualified. Witches were supposed to turn into vampires after they were buried, as also was the corpse of anyone else who burial had been jumped over by a cat or flown over by a bird before burial (Wickwar 134).
Victims of suicide and those who did not receive a Christian burial were ready candidates to become vampires. Additionally those whose murders had not been avenged, evil doers, and even those born on Christmas were likely to suffer the same thing (Hill 2926).
Vampires could also take on many, many different forms, such as flying insects (especially moths or butterflies), crows, mice, and werewolves (Strigoi 1). Strigoi mentions that the Serbian believed that vampires could transform into butterflies. The Balkins had nearly the same belief. Strigoi went on to say that a black hen, or a bat crossing over an unburied corpse would cause that corpse to become a vampire (1). The Slavs' version of the vampire was not nearly as beautiful or graceful as a butterfly. To them, vampires were little more than a head with some entrails hanging from it (Wickwar 143).
So, now that we know a beginning about the vampires of myth, how do we stop them? Well, everyone has heard about sunlight, crosses, and garlic, but why were these "cures" chosen, and were there others?
Corpses could not become vampire unless they were buried, or so said the folklore, and also could not rise if they had a heavy headstone. But you could not leave that many bodies above ground, and you may not be able to get headstone. What did you do then? Some other solutions were to dig up the grave of a vampire in daylight, put a stake through it's heart, or shoot them with a silver bullet (preferably one that had been blessed). One could also kill them by showing them a crucifix, burying them under running water, or at a crossroads (Hill, 2924), or any number of other tactics. There were many ways of finding vampire graves. After the grave is found, you can kill the vampire by the old stake in the heart method. One Hungarian way of finding graves follows: "a white stallion that has never stumbled and has never been out to stud is taken to the cemetery; the horse will refuse to walk on a vampire's grave (Hill 2925)." Also, vampires did not have reflections, so upon seeing a "man" with no reflection, you could follow it to its grave and kill it when the daylight came.
Vampires Cannot Exist in Sunlight
"Vampires were believed to be servants of Satan; they were demons capable of all types of trickery." Because darkness was synonymous with evil, and light with good, it would seem that vampires could not exist in the light. Churches believed that nothing is stronger than God, and even a vampire could not survive under God's light. This holy/unholy reasoning was also why vampires could not walk on holy ground or enter a church(1).
Crucifixes and Holy Water
Again, God is good, vampires are evil, holy water and crucifixes are holy items, both blessed, one also an image of Christ. The holy water burn s the vampire in cleansing the evil(1).
No Reflection
A common belief was that vampires did not have souls. The soul was the part believed to be reflected in the mirror, so if vampires have no souls, they must also have no reflections. An interesting idea that NightPoe brings up is that Catholics also believe that animals don't have a soul because they are not human, yet animals have a reflection(1).
Four Ways to Kill a Vampire
1. Stake Through the Heart There was, at the times when vampire stories were most prevalent, a disease called consumption that plagued small towns and villages. This disease literally consumed the body, as a vampire drawing blood over many nights would. It was believed that unless you pinned the corpse of one of this disease's victims down in their grave, they would come back and infect the living. Corpses were pinned down with wood because there were no metal nails available. This disease is still around today, but is treatable. It is known as tuberculosis(1).
2. Drowning in Running Water Again, water purifies, and running water more than other(1).
3. Burning This originated from the Christian belief that fire purifies all evil(1).
4. Beheading and Removing the Heart The evil was considered to be held in the head and the heart. If those two organs were separated, the evil could not continue to exist(1).
Headstones
Headstones were needed to keep the dead person from crawling out from the grave(1).
Vampires Need an Invitation
There are two possible causes for this one. The first is that people were scared and needed something to keep them from completely giving up because they felt nothing could help them. The second was that there was so much life energy in one household that a vampire could not enter and move freely though it unless invited (1).
Some old vampire stories are stories of opening graves to find pink not yet decayed flesh, days after they were buried.
Most myths such as this (though they should not be called myths, they actually happened), were started by finding the body turned on its side, curled up, or with blood on its hands or face. At the time these myths began there were not good doctors, and many people were buried alive. Upon awakening in their graves they would try to escape. Their shifted positions were caused by futile attempts to get out of their coffins. The blood on their hands was caused by them trying to claw their way out of their coffins, and the blood around their mouths was from death (Hall 2926).
Wickwar related a story of a doctor of medicine in Iowa who exhumed a corpse to find that the coffin's joints had broken and that hair fell from the openings. The doctor had evidence that the man's head and face had been shaved prior to the burial, four years before, yet the hair on his head had grown to 18 inches, and that of his beard was 8 inches (155).
That explains the "undead" that were found rosy checked and that screamed and bleed upon staking and beheading. It also explains some of the pieces of movies like Dracula or Interview with the Vampire. But was there a real Lestat? I don't know about that one, but there was a real Dracula.
Hope this helps