Regretfully, I must return to Real Life. Just assume that Vader protects the Witch-King (and Pyro, obviously) until they die or Amilia and the Uruks are defeated. Or Vader has a Dramatic Last Stand and dies epically.

EDIT: Oh goodness no, I was [u]protecting[/u] the Witch-King. He's a useful ally [i]and[/i] the only way anyone's getting out of Angmar alive.