In the untainted version, Palps contacts Vader when Luke pursues Jedi training. Since Palps doesn't mention the Death Star, it was always pretty clear to me that Luke had just graduated from being a thorn in the Empire's side to being a genuine threat to Palp's power. Palps knew who the kid was, but didn't fear Luke until he started training as a Jedi.

Palpatine contacting Vader was his way of saying, "Step up the chase, Vader! This kid could destroy us." Vader makes an argument to convert Luke so the boy can live.

In the new version, it's firmly established that this is the first time Palpatine and Vader have ever spoken about Luke. That's why it's at odds with the rest of the movie (particularly the opening crawl).