[quote=@Keksalot]
Yeah, rivalry and hatred, if there's any at all. Maybe it's just healthy competition of two breeds of beasts in one world, without any emotion involved? Or is there like, mutual disdain or something like that? Werewolves could've hunted vampires because they, being deathless and undead are atrocities and insults to the natural order of things, for example.
[/quote]

They are rivals because, due to other lore from movies, books, etc. we have learned that they are natural born enemies, and yes, that is a good reason for it. Besides, the Pack is now infiltrating vampire land, thus already making vampires angry, due to the say... being territorial.