I'm liking the idea that Vader actually knows about Luke, or at least knows he's alive somewhere and that destiny will eventually lead Luke to confront his father. But Vader wants this so he can try and lure Luke to the darkside ala ESB.

But Vader shields Luke from Palpious so he can use his son against the emps at some later time.

It explains why Luke is on Tatooine and Vader never bothered to look into a Tatooine phone book under Skywalker.

He already knew he was there. But was waiting until the right time when his son would confront him and then he could try and lure him to the darkside in ESB. Then he would be powerful and also capable of helping defeat the emperor. Then they could rule the galaxy as father and son.

And if you look at the existing films, there is a point where Vader says Obi-Wan was hiding Leia. But at no point does he say the same about Luke. In fact, Vader seems to know a bit about his upbringing.

It also plays deeper into Vader's own desire for power and control that he mentions in the PT.

So maybe Vader was actually shielding his son from the emperor until post-ANH when the secret was out in the open. That's when the emperor says he have a "new" enemy.