But throughout the first SW series, we are led to believe that Vader is completely engulfed by the Dark Side. In fact, a little bit of Anakin had survived somewhere within him. I think Luke must have sensed this - I don't remember - and tried to sway Vader back to the Good because Vader was his father. In no other fiction that I can think of was the major villain in any way related to the hero.
In any case, if there was even a bit of Good in Vader, then Vader deserved to be redeemed because there was still hope. A hero always lives on hope, and that's another reason why Luke was the archetypal Hero.