I have always thought that Palps & Vader have known of Skywalker and who he was since the destruction of the Death Star, Vader becoming obsessed in finding him as the opening crawl says.

Palps urgent conversation with Vader is because suddenly things have changed, there is a disturbance in the force, ie: for nearly 20 years the Jedi have been extinct and now all of a sudden these 2 Sith can feel tremors in the force, as Yoda has returned and Luke is getting stronger, like flies caught on a spiders web.

If you think of it this way....everything makes sense.......but not if you listen to the new SE dialogue.......that really throws everything into the contradiction zone!