Horrible creatures that stay up all night drinking blood exist in stories from all over the world. Every culture ever has some sort of demon or creature that steals your blood in the night. The most popular version is the vampire of Eastern European folklore. Nearly every modern vampire story is built upon European stories of revenants that rise from the dead to drink the blood of the living.
Modern fiction plays with the vampire mythos to create versions that don’t quite jive with European folklore, but most stories use the same set of weaknesses. These methods used to kill a vampire do as much to define the modern vampire as any other trait.
Vampires are crazy photosensitive
Sunlight is pretty much the worst thing ever. You don’t really have to be an undead monster to realize that going outside when it’s sunny is pretty much the worst thing ever. Sunburn is painful and sunstroke can literally kill a normal human being.
This trait was built into the stories to boost some of that “creature of the night” suspense. Most vampires are killed by sunlight, but there are many versions where the sun is just a nuisance. In fact, Eastern European folklore really doesn’t talk about the sun being painful to vampires at all. They’re basically just nocturnal. Some versions talk about the moon increasing the vampire’s power, like some sort of reverse Superman.
The sun becoming a deadly force in the fight against vampires probably developed as a plot device. If you take a bunch of people fighting off vampires all night long and give them “sunrise” as a goal, you can escalate the danger as much as you want and people will believe that they “made it out alive” for a reason more than just dumb luck.
Wooden stakes are basically the best way to kill a vampire
You can kill a vampire by striking them through the heart with a wooden stake. Buffy the Vampire Slayer relied heavily on the wooden stake. Probably because it worked with the “silly little teenager fighting off horrible creatures” thing it had going for it. The girl was unreasonably unmatched and she basically used a stick as a weapon.
This part of the vampire mythos is rooted heavily in some of the real world fears people have surrounding the dead. It turns out, when a dead body is left to rot, they can simultaneously bloat up and turn red while also turning pale. They’ll lie in their coffins getting bloated and filled with (if you were to bother to check inside of them) putrefied blood. The gums will recede and the skin pulls back just enough to make it look as though there’s blood coming out of some of the important face-holes.
So some slightly less enlightened people of the “good old days” would see a dead body of a person who was a bit horrible in life and just assume that they had gotten up in the middle of the night and gone out to drink the blood of their family and friends. The only way, they decided, to keep the things in their coffins, was to drive a wooden stake through them and hold them in place in their coffins.
Of course, some people had other ideas.
You’re totally going to need your head to drink blood
Beheading a vampire works because it’s totally hard to do anything without a head. At least, as a function of story, a headless vampire would be more silly than scary. Groping around, holding it’s head in its hands. Silly, silly vampire.
Some of the same folks who thought that they had to jam a stake through the hearts of the dead to hold them in place decided that they could keep the vampires from escaping in the night by chopping off their head. Many of these same people decided that they would jam the vampires heads under their butts for some reason. Possibly as a “screw you” to the unpleasant dead person.
Even the living dead don’t want to be burned alive
You having issues with the dead rising from their graves and sneaking out to feast on the blood of the other villagers? No problem. Just burn them to smithereens.
Thing is, this plan isn’t so bad. When you’re dealing with rotting and decomposing flesh, you’re dealing with a possible cesspool of bacteria and disease. Tons of cultures have burned their dead. While it is a great way to completely obliterate a vampire, it’s actually a reasonable practice. The body can’t infect people with filth if you turn it to ash.
And then all the other stuff too
Silver is a common tool to kill vampires, but it is a bit more recent. European Folklore doesn’t really cash in on this one, actually. It’s more common in more modern versions (read: North American versions), and probably has something to do with other myths that involve silver. Some of this stuff gets lost in translation over the years.
Holy symbols are tried and true, but rarely considered fatal. It’s great to be able to have a tool to keep the vampires at bay so you can get into a nice little chat. It doesn’t always have to be fight-to-the-death, sometimes you can just hold up a cross and chat with the monster that’s going to kill you. Works great to make room for exposition.
Garlic is stupid. It’s totally legitimate and routes all the way back to the earliest tellings of vampire stories, but it’s hard to make it sound cool.
There’s about nine thousand variations on what works and what doesn’t, but you’re not going to make any friends by playing around with these conventions too much. You can make it in and out of a story without really getting caught up with the whole garlic thing. No one’s going to be super upset if the holy symbol doesn’t work for some reason – especially if they’re doing that “meta” thing where there’s vampires in a world where people already know about vampires. But when you mess with the stake/sun/beheading thing too much, you start to mess with people’s suspension of disbelief.
And vampire’s aren’t even real.