Yeah, I think Vamps are just always sprinting everywhere; not sure it has anything to do with FEAR.
Anyway, legstabs are necessary if you want your mark. DON'T ACCUSE OF BEING A NIGHT CREATURE. If they admit to being the person who's a Vampire, just fucking get the drop on them--they'll be halfway across the fastravel tile before you hear them shouting back about the many foes they've vanquished in their lust for blahblahblah.
Take a throatshot if you're feeling confident in your ability to hit, but even then I've had freaking goblins take a throat shot from a silver pike, have it open arteries and chip the spin\tendons, and they kept running--so legs are perhaps the new gold standard. Additionally, I noticed more people knocking out from pain due to leg stabs than neck stabs.
Vampires, being dead, don't tire out at all. Thus, they're capable of sprinting forever until they think it's safe or they get stopped by something. They also don't breathe, don't age, don't feel pain, can't be stunned, don't suffer fevers, don't become nauseous, don't grow dizzy, and don't even have to sleep. To that end, you should take out their legs as fast as you can or else they'll sprint off into the night. You'll literally never catch them until they stop running unless you're a vampire as well, a necromancer, or some kind of creature that also won't tire out.