There's a great interview with George Lucas in Rolling Stone magazine this week (#975: 2 June 2005). In it, he reveals Vader's supposed inferiority as villian and his relationship with Luke.

RS: Even as you were building up this iconic villian, you knew the tragedy behind it.
GL: He's so overwhelming in the first film, but you get to the point where you say, 'Wait a minute, if he's so powerful, why doesn't he run the universe?' He even gets pushed around by the governors! They know the Emperor is the final word, so what happens is the same thing that happens in any corporation: Everybody worries about the top man, they don't worry about his goon. . . . So it's even more tragic, because he's not even an all-powerful bad guy, he's kind of a flunky.
RS: He's not Satan, he just goes down to the corner and gets Satan's cigarettes.
GL: You got it. And when he finds out Luke is his son, his first impulse is the figure out a way of getting him to join him to kill the Emperor. That's what Sith's do! He tries it with anybody he thinks might be more powerful, which is what the Emperor is looking for in the first place: somebody who would be more powerful that he was and could help him rule the universe. But Obi-Wan screwed that up by cutting off his arms and legs and burning him up. From then on, he wasn't as strong as the Emperor - he was like Darth Maul or Count Dooku. He wasn't what he was supposed to become. But the son could become that.

Aha! So the Emperor (for lack of a better term) 'recruits' Luke because Vader is unable to live up to potential. Interesting. I suppose Vader was unaware of this; his motivation for offering Luke the chance to replace the Emperor is merely an effort to reestablish his own presence as Chosen One. Of course, Vader's characterization is a manifestation of deeds and not desire. Meditate on this, I will.
http://www.rollingstone.com/news/story/_/id/7314866