Lost Boys: The Thirst
(2010)

Edgar Frog, his sidekick Zoe and dumb reality star Claus are hired by lousy Vampire-Romance writer Gwen to rescue her brother Peter from the leader of all vampires. Edgar hopes that killing the leader will save his brother Allan who got turned.

Claus gets killed gruesomely but the rest manage to kill the leader with he help of Allan.

Turns out Peter is the true leader whose power the other wanted to steal. Edgar splashes ordinary water on Peter and blesses it to make holy water. Peter dies instantly and Allan is saved. With no vampire in sight Edgar decides to look into other supernatural creatures to kill, and especially werewolves, not knowing that Zoe is one.

Thanks Anthony L!