Queen of the Night Time World — Kristen Strassel

Holly never believed she'd lose Rainey, until she did. Rainey has failed her mission on earth, and the creature who sent her wants her back.
Holly refuses to accept it. Rainey is the balance that keeps the universe in check -- good and evil, sin and desire, dark and light. Without her, the vampires in Vegas will drag the city straight into darkness, and Holly will fall with them.
The fate of the city depends on Rainey, and Holly must convince the vampires that without her, not even their immortality will save them. Because even in the depths of the Las Vegas underworld, love is stronger than evil, and light is more powerful than darkness.