What is the point of it, though? You don't get XP from killing another vamp, so it's more of an annoyance than anything else. At least human TKers provide more zombies/vampires. TKing vamps/zombies doesn't do anything, unless you're a different race.
Also, if we're getting TKed, I don't see any problem in retaliating. I don't think the rule against it should apply to self-defense.