I have no idea if any of these quotes help but here you go.
"Anakin, as Skywalker, as a human being, was going to be extremely powerful, but he ended up losing his arms and a leg and became partly a robot. So a lot of his ability to use the Force, a lot of his powers, are curbed at this point, because, as a living form, there’s not that much of him left. So his ability to be twice as good as the Emperor disappeared, and now he’s maybe 20 percent less than the Emperor. So that isn’t what the Emperor had in mind. He wanted this really super guy, but that got derailed by Obi-Wan. So he finds that, with Luke, he can get a more primo version if he can turn Luke to the dark side."
--George Lucas, “Star Wars: The Last Battle,” Vanity Fair, 2005
"At this point, Vader’s plan really, now that he knows he’s his son, is to convince him to come with him. Join the dark side and together they’re going to overthrow the Emperor, which is the thematic devices used through the whole movies in terms of the Sith, which is Sith Lords are usually no more than two because if there are three, then two of them will gang up on one to try to become the dominate Sith. Anakin would have been able to do it if he hadn’t been debilitated and now he’s half machine and half man, so he’s lost a lot of the power of the Force, and he’s lost a lot of his ability to be more powerful then the Emperor. But Luke hasn’t. Luke is Vader’s hope. His motives at this point are purely evil. He simply wants to continue on what he was doing before which is get rid of the Emperor and make himself Emperor. He only sees his son as a mechanism for the ambition. His mad lust of power."
--George Lucas, TESB DVD Commentary.
"And when he finds out Luke is his son, his first impulse is to figure out a way of getting him to join him to kill the Emperor. That's what Siths do! He tries it with anybody he thinks might be more powerful, which is what the Emperor was looking for in the first place: somebody who would be more powerful than 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."
--George Lucas, Rolling Stones Interview, 2005.
"The Emperor wants Luke to kill Vader so that he will have a new young Jedi. Lets face it Vader is half mechanical and he is not half as good as he could be. He is not nearly as good as he was hoping Anakin would become because Anakin ends up in the suit. He is hoping he gets a new better apprentice in Luke. If he kills his father then he would take his place as an apprentice; which actually there is something that in the next film is how Anakin becomes his apprentice."
--George Lucas, ROTJ DVD Commentary.