Ask HN: How do vouching privileges work on HN?

Sometimes I see a decent post that's "[dead]" simply because it's a new account and even if I vouch it, it continues to be dead.

Do many people need to vouch it for it to become undead? How many karma points are needed for vouch to work at all? I suppose if the option is there, it should work but... there must be something more to it.