When Cordelia wished that Buffy had never come to Sunnydale, her wish is granted by Anyanka, a vengeance demon, who casts a spell that creates another dimension where Buffy has indeed never set foot in Sunnydale (season 3 episode 9 The Wish). The town is infested by vampires, led by The Master and his minions, Xander & ... Willow, both vampires.
The vampire Willow has a liking for leather and low cleavages. She's Xander's girlfriend although she seems pretty open for buisness with anyone, especially girls, and has trouble keeping her hands to herself.
It's never clearly stated, but Willow was mostly vamped during 'The Harvest' (see BtVS season 1, episode 2). Buffy not being around, the Master and Luke, his vessel, were able to break the Master's mystical prison, allowing him to walk free on Earth. Willow was kidnapped by one of the Master's minions, Thomas, and fed to the Master.
Once she was vamped, she most likely went after her childhood sweetheart Xander and made him hers forever. With no Slayer to protect Sunnydale, The Master and his kind were free to take over the town.
Willow is ambiguous and puts on a naive & innocent act, ressembling a somewhat spoiled child. Her favorite phrase is 'Bored Now', generally followed by a brutal killing. She is sadistic and enjoys torturing 'Puppy' - Angel, captured by the Master. She also mentions enjoying the chase and is truly gleeful when omes the kill. She seems to consider being a powerful vampire a sort of game where she sets the rules and plays the pawns.
Vampire Willow is killed in the alternate dimension by Oz himself, who drives her onto some broken wood, turning her to dust without a secnd thought.
When the wish is broken, Anyanka looses her powers. Furious, she poses as a Sunnudale High student and enlists a frustrated Willow into her helping her with a spell (season 3 episode 16 Doppelgangland) - forgetting to mention that she wants to get her demonic powers back. The spells goes wrong when Willow catches a glimpse of the Hell universe. Instead of returning Anyanka's powers, the spell brings the Vampire Willow into their dimension.
As she walks around town Vamp!Willow realises that she is not in her world. As she enters the Bronze she encounters a living Xander and Buffy and Xander think she's the real Willow and that she's dead.
Vamp!Willow easily submits a gang of vampires and leads the to take over the town, starting by the Bronze. There she runs into Anya, who realises who she really is. She explains that this is not her world and that only the other Willow, the human one, can send her back to her world. Vamp!Willow leaves her new croonies to guard the Bronze as she goes to get Willow at the school. Willow is shocked when she sees herself all evil and Vamp!Willow seriously seems to consider vampire herself, liking the idea of having two of them. Willow manages to shoot her down with the tranquiliser gun the gang use on Oz. The scoobies turn up and shut Vamp!Willow into the bookcage. They then head over to the Bronze, the real Willow posing as Vamp!Willow. While the gang take the vamps at the Bronze out one by one, Vamp!Willow convinces an unknowing Cordelia to let her out of the cage, proceeding to chase her round the school. Cordy manages to get away with help from Wesley and Vamp!Willow head over to the Bronze. She attacks Willow and Buffy almost stakes her, stopped by Willow, not comfortable in seeing herslef staked. They decide to send Vamp!Willow back to her own dimension, just seconds before she is pushed onto shatted wood by Oz, who dusts her.
Despite being in only two episode, Vampire Willow imeadiately gained a large fanbase and lives on in many fanfiction stories. We also catch a little glimpse of vampire Willow when Willow turns bad at the end of season six, although she is far more anger driven that Vampire Willow, but she also uses the infamous catch line 'Bored Now'.
Credit : Emma-Jane