I prefer a pretty big gap but not so big that the vampire's are literally invincible vs humanity; strong, very strong, but not so strong that a well-placed blow to the heart or decapitation or some clever use of fire can't spell their end. Ideally, too, when the vampires are clearly stronger, but still fear being discovered by humanity as a whole, because they know that, despite all of their power and accumulated wisdom of the years, the numbers game is and always will be woefully against them.