(sigh)
Okay, listen carefully. Can you hear me in the back? Good...
1. Droids and Obi-wan: In ANH, when Obi discovered Luke and the droids in the Jundian Wastes, he was still intent on keeping Luke's lineage a closely-guarded secret. Any revelation of the origins and history of the droids could have led to an awkward series of questions from Luke, which, at some point, would have become very difficult to explain. Or, in the course of those explanations, Luke may have eventually become suspicious. Better to leave Luke completely in the dark by allowing him to believe that Obi-wan, too, is completely in the dark.
2. Emperor in the Dark: At the beginning of Empire Strikes Back, we can assume that the hunt for the Rebels, particularly Luke, has been a long, ongoing quest for the Empire by this point. Yet, by the time we reach the asteroid field and Vader is summoned to an audience with Palpatine, the Emperor gives Vader (and us) the impression that he has only recently discerned that this "new enemy" is the offspring of Vader. Ergo, we can assume that this Imperial quest for Luke has been underway all this time
without the Emperor's knowledge. Vader's keeping it on the downlow because of his own hidden agenda.
Vader's own dodgy comments in his audience with the Emperor lend weight to this. He continually tries to deflect Palpatine from ordering the boy's death. To wit: "How is that possible?" "He's just a boy. Obi-wan can no longer help him." "If he can be turned,..." blah, blah, blah.
Frankly, I'm pretty sure that, once Vader discovered that Palpatine was onto Luke's existence, he pretty much crapped himself and was hastily trying to salvage his mission by keeping Luke alive. Whether he was doing this out of love for his son, or some darker, sinister agenda, is a subject for another debate...
