The very last works if you're doing something in modern times. What I've read about vampires is that a normal knife will hurt them, but they can recover from it quicker than something made out of silver can.
Generally speaking, silver kills werewolves and vampires are killed by being stabbed with wooden stakes or being burned in sunlight. But that's the world's mythology. Maybe your vampires aren't like the ones in the widely accepted mythology, or maybe your silver knife is special for some reason. I think if you're the one creating the world, it gets to be your call.