In the ESB and RotJ novelizations, Vader was indeed sincere. He wanted his son by his side, and even had some measure of affection for him, twisted though it may have been. His temptation of ruling the galaxy as father and son was exactly how he envisioned it. In the novels, right up to where Luke beats him in RotJ, Vader is planning how to get Luke to fully fall to the Dark Side, and pledge loyalty not to Palpatine, but Vader.

Outwardly, Vader's motives would have been more single-minded; for a thousand years, there has always been a master and an apprentice (the "Rule of Two"). This rule is self-enforcing; one Sith is too easily killed, ending the Sith forever, but with three Sith, two of them will invariably gang up on the third. This happened with Darth Plagueis; his student, Palpatine/Darth Sidious, had been training Darth Maul in secret, and rose up and killed Plagueis when Sidious felt he could learn no more from him. Vader's been in Palpatine's shadow for nearly twenty years; it's high time Vader became the master.