Out for Blood (2004) on Netflix

In Los Angeles, Detective Hank Holten is obsessed by his ex-wife, the writer of vampire novels Susan Hastings, stalking her and having drinking problems. His chief and friend, Captain John Billings, makes him promise to forget her and stop drinking, and assigns Hank to find Layla Simmons, who is missing and without notice to her family. Hank goes to a rave party, finds Layla, they go to an orgy party in an old building, and while having sex with her, Hank finds that the place is a vampire nest. He is bitten by the vampire-leader, but nobody believes on him. He looks for Susan to help him.