It all began with just a simple wish...
Cordelia Chase, unreasoningly angry over the turn her life had taken , blurted out, "I wish that Buffy had never come to Sunnydale!" Sadly, she did so with a vengeance demon called Anyanka nearby, who granted that wish, and remade their world.
Sunnydale was transformed into a vampire ruled city, lead by the Master. Willow and Xander, once heroic champions, were vampire servants of the Master, torturing Angel for their amusement. A rag-tag band of vampire slayers lead by Rupert Giles tried to oppose him, but their efforts were mostly futile.
Calling upon a scarred, bitter Vampire Slayer named Buffy they prepared for a final battle even as Giles worked to undo the wish that had created that world. The Master slew Buffy with his bare hands, even as the vampire slayers fell all around them, Willow being slain by Oz, even as Giles shattered the crystal of power belonging to Anyanka.
Sunnydale was restored to normal, and that should have been the end.
Anyanka, now simply calling herself Anya, desperately wanted to regain her power. Calling upon Willow wiccan assistance she tried to reach into that 'wish-verse' to retrieve her powers. They failed, but unknowingly they had brought something back with them.
The vampire version of Willow had come to Sunnydale. Gathering hench- vamps, she sought to make this world like the one she had come from, but Buffy and her friends managed to round her up fairly quickly. But the vamp Willow was a disturbing revelation to the merry band, with her clear interest in the female of the species and kinky sexual nature.
With Giles' assistance Willow reversed the enchantment, and after getting in a bit of a grope of her counterpart, vamp Willow went on home. Willow's boyfriend Oz left, and after a period of grieving Willow fell in love again, this time with a woman named Tara. Their lives went on, their love was challenged, broken, and ultimately restored before Tara was tragically slain.
In the fanfic Buffy the Vampire Slayer: Itsumo means Forever, Willow has returned to Sunnydale to guard over the First, who was sealed away behind the Hellmouth by Buffy, Spike and Anya's heroic sacrifice. Still in shock over the return of Tara, the servants of the First conjured up the vampire Willow, hoping she could slay her twin.
While Willow discovered her combat magic's couldn't directly effect her vampire twin, she could quite effectively contain her. At Faith's suggestion they were able to restore the soul of the vampiress, and then returned her home, unknowingly setting a new chain of events in motion...