According to fiction, vampires does not need to have sex to reproduce.
Read Breaking Dawn by Stephenie Meyer
Stephenie Meyer's Vamps: They can but they can't concive a child unless it is an human femal and vamp male, only vamp males can reproduce (with humans) but because vamps can't change female vamps are unable to reproduce
Or else, in traditional vampire folklore, vampires are very sexual and seductive creatures. They do not need to sex to reproduce, but they do it because they like it.