From the very beginning, since William Corvinus (the first Lycanthrope to walk the earth), the werewolves were enemies of the vampires, as well as all other werewolves tribes who were scattered around the globe.
Lycanthropes and Quileutes had many points in common: two of which included they were werewolves and enjoyed killing vampires. So what would be more rational than to think that they would unite against the common enemy, the vampires? Alas, that's not what happened.
Some generations of Lycanthropes (or just Lycans) were so primitive that they stayed in animal form for years feeding an irrational rage, forgetting their humanity and often acting without considering the consequences. In these inconsequential attacks, another enemy was made because a Lycanthrope had no justifiable reason to kill a Quileute wolf.
Since these unjustifiable deaths, the two tribes have created an animosity that has lasted for centuries and to be able to coexist without war they had to establish limits, rules. The forest was divided in half. On one side, the Lycanthropes and the other Quileutes and neither could trespass on the limits set. If they did they risked attack and certain death.
These limits and rules need to be respected even when a vampire invaded the territory. If a Quileute chased said vampire into the Lycan's side, said Quileutien would not and could not continue the pursuit leaving the vampire for his enemy to kill.
These rules and limits were set centuries ago and persists to this day.