"The best way to show this is with a vampire bat and a real wolf. The vampire bat will try and feed on the wolf because it needs blood to survive.
The wolf will not like that anyone knows about it. For other reasons, a bat is a nice morsel to eat that even comes to it rather than being hunted. To each of them, the other is prey. There is no ancient blood feud going on.
Vampires feed on humans and relatives, then move on to friends and others in the town or even the city.
And, generally, they attack night after night, slowly draining the victim of life.
Of course, they also tend to wear the victim out too. More than one vampire has been associated with sex, coming back from the grave to visit his widow until she eventually succumbs to death. That is basically like the plague. One victim falls ill, then another, then another, and so on.
The werewolves, however, tend to kill outright. They eat the victim or parts of them. And, generally, they prey on weaker victims, such as children and women.
They prefer isolated places where they can kill, feed, or at least drag the victim off to eat without being discovered.
Now, if they meet, it will depend on what form. Usually, the vampire will inhumanly feed on a sleeping werewolf. The werewolf will attack and feed on the "sleeping" vampire in the grave.
The werewolf might attack a physical vampire (body), but this would be rare, as they are not seen, the way human shows them.
Think more of a ghost or spirit that the wolf could not bite. That would make it extremely hard for a werewolf to defend itself, too. Not exactly a fair fight.
There is also the fact that some legends say that you become a vampire after being a werewolf. In other words, the werewolf dies; and then, becomes a vampire, continuing to prey on the living, even in death."
For these two reasons, vampires, and werewolves, will be forever enemies and will never accept the union between their clans and will do whatever it takes to avoid it.