1. No one can ever agree on vampire powers, so a werewolf would be entering into a one hell of a gamble if he ever picked a fight with a vampire. It could do anything; transform into a tiger, or spew acid from its eye sockets. So the first reason is: uncertainty!
2. Werewolves are werewolves for one night a month. Vampires are cold hard bloodsucking bastards for hundreds of years, every day, and all hours. Except maybe the daylight hours.
3. Vampires are smarter. It’s true, sorry, but werewolves are mostly created from the “let’s go investigate scary noise” horror movie stock or, “ignore the crazy gypsy woman” types. Vampires are created either by choice, or because they were just so damn sexy.
4. Vampires are faster; they retain human forms, and don’t become huge, ungainly half-man half-wolf creatures that have to re-learn how to walk. Animal reflexes are on the werewolves side, but supernatural speed, –nope.
5. Werewolves are usually crippled by a lengthy, painful transformation, giving a vampire ample time to smoke, check his laundry, eat a light dinner of blonde virgin, watch the latest episode of True Blood on HBO, come back after picking up his dry cleaning, and finish on the werewolf before he’s even finished growing a tail.
6. Vampires are more popular; they’re hot, and they look like gorgeous pale people, thus, they’re more likely to have human allies, willing to die for them. Werewolves are scruffy, un-hygienic looking guys, who might have one girlfriend that they end up killing off themselves.
7. It’s easier for vampires to blend in with humanity, giving them an edge when it comes to hiding, or using the element of surprise in a fight. Werewolves are huge, mutant dog people; they don’t really have the same thing going for them once they’ve transformed.
8. Vampires multiply faster; it takes one night for a vampire to turn like, a dozen or so people. It takes a whole month for a werewolf to turn someone, or at least for the effect to kick in.
9. In the event that a vampire should have to flee, because the werewolf is fighting dirty, or ganging up on one vampire, then vampires can always shapeshift into something with wings, like these so-freaking-horrfiyingly-ugly-they’re-cute vampire bats. Then they can come back later and snuff the werewolf when he’s in the bathtub or something.