Almost nobody in modern America believes in vampires anymore - but we almost all agree on what properties a vampire has. Mostly, we base these assumptions on modern portrayals of one character: Count Dracula.
The name "Dracula" (Romanian for "little dragon") was borrowed from the epithet of a real Wallachian prince, Vlad III (also known as "Vlad Tepes," which famously translates to "the impaler"). However, the fictional count and the historical prince are not particularly similar to one another in any way, except for both being of an aristocratic background and living in eastern Europe. Bram Stoker, who created the vampiric character, simply liked the menacing sound of the name.
Stoker's Dracula draws rather heavily on a jumbled mess of a book called Varney the Vampire, or, the Feast of Blood. Varney established the Victorian image of the aristocratic vampire, which persists today. (Exactly what Varney's backstory is is never spelled out in detail, but the author gives clear hints at some contradictory answers.) Stoker's twist, however, is to establish a coherent set of powers his vampire has, and limitations that apply to them. Stoker's Dracula can shapeshift, becoming a bat, a wolf, a dog, or mist (with no explicit statement that these forms are his only possibilities); he drinks blood and has trouble controlling his hunger for it; those he bites become vampires upon their deaths (which are often immediate, as he drains enough blood to be lethal); he's able to control those vampires he created. He loses his powers by daylight, though it isn't harmful to him.
Significantly, the portrayal of vampire attacks and of the almost disease-like portrayal of the deathly sleep of the count by day recall images of disease and sexuality, almost presenting vampirism as a venereal disease. This plays perfectly on the anxieties of Victorian society.
Most modern portrayals of vampirism have limited the shapeshifting to a bat, possibly because it's the only thing he transforms into in several Hollywood adaptations of the story. (Others don't even show that, and many other modern fictional vampires aren't able to shapeshift at all.) However, there's a single adaptation that has defined the greatest weakness of most present-day vampire fiction - and that one was a flagrant copyright violation.
In 1921, German expressionist director FW Murnau decided he wanted to make a movie out of Dracula. Denied the rights by Stoker's estate, he simply renamed all the characters, with Jonathan Harker becoming Thomas Hutter and the count being renamed Orlok. The film was released the following year, under the title Nosferatu: a Symphony of Horror.
Nosferatu contains one major deviation from Dracula thematically. Weimar attitudes toward sex were quite different from Victorian ones, so Graf Orlok takes on a hideous form rather than putting up an affable, aristocratic front like his literary precursor. The image has been compared to a plague rat, and like disease-carrying vermin in the popular mind, Orlok is never out in the daytime. It's a sensible choice for a world only four years past the outbreak of the "Spanish flu," the most devastating pandemic of the past several centuries.
Although the gentlemanly vampire who hides his monstrosity under an outwardly charming front remains a mainstay of modern vampire stories, as does the sexual quality of Stoker's presentation, one thing about Orlok has caught on: his fatal vulnerability to sunlight. Instead of the ending following the novel, Orlok is ultimately destroyed when he is so overcome by his bloodlust that he keeps feasting as the sun begins to rise outside, its light burning him to ashes.
Consider now a typical modern fictional vampire, such as those in True Blood or in the roleplaying game Vampire: the Masquerade. While some individual vampires may be brutish in appearance or demeanor, or even have the hideous aspect of Orlok, the most typical portrayal (and, fairly reliably, the first we meet, as in True Blood) is a debonair and charming person who just happens to live on human blood. These characters only come out at night, however, not merely preferring the darkness as in Dracula but depending on it as in Nosferatu. We've also accepted that certain weaknesses derived from earlier European folklore will be either included or derided, depending on the author's preferences: vampires may shun garlic or crosses, be vulnerable to wounds from silver weapons, or cast no reflection in mirrors.
The hideous, plague-oriented vampire of Nosferatu also thrives in modern popular culture. The novel I am Legend (Richard Matheson, 1954) deals with a world overrun by such vampires, their condition spreading like a plague (it's actually caused by a bacterium) and the sufferers taking on a hideous, rotting quality. For pseudoscientific reasons, these vampires share some of the folkloric weaknesses of the species, including the one introduced a mere three decades earlier by German cinema: they're vulnerable to sunlight. (Most of them are also repelled by crosses.) Matheson's vampires are seemingly mostly mindless, and are driven to prey on the uninfected.
The image of a world overrun by a disease of undeath has inspired other horror authors, particularly the filmmaker George Romero, but we've abandoned any notion that this type of monstrous apocalypse should tie into vampire mythology in the slightest. (The word of choice is usually "zombie," despite also having no connection to the Afro-Caribbean legends from which that term is derived.) Most portrayals also don't embrace Matheson's powerful ending, in which it turns out that the monsters the protagonist has been killing throughout the novel were still fully aware of their surroundings and were slowly building a new sort of society that works with their condition. The title derives from the realization that, far from being beset by monsters out of ancient folklore, he has become the monstrous killer who will haunt their legends for generations to come. Instead of this, however, most versions of the zombie plague motif simply have mindless hordes of the shambling dead - an end of the world, not a transformation. Their gruesome appearance and all-consuming hunger for the living, however, reflect an attitude toward death and disease born out of a terrifying 20th-century plague, and so they remain inheritors of a vampire tradition.
As our societal reaction to death and disease changes, so do our undead - from the revenants believed to stalk England in the high middle ages to the slow wasting away of the victims of an early American vampire to, ultimately, the seductive aristocrat and the rotting hungry corpse that reflect two very different streams of the tradition in modern western horror fiction.