Otter's Rock

Here is a Star Wars question for you guys!  And I hope someone who's been reading the new comics can tell me if there is a canon answer for this. When Darth Vader learned about Luke, how much did he find out?  Like, did he think that Obi-Wan had been raising him as an apprentice, or did his agents go to Tatooine and dig up that Luke had been raised by his aunt and uncle on their moisture farm?

Title: Resonance
Author: beatrice_otter
Fandom: Star Wars
Rating: PG
Characters: Darth Vader
Word Count: 478
Written For: imadra_blue
Prompt: So, what happened in ESB between Leia and Vader when she was tortured? Why did he keep changing his mind and insisting on bringing her with him?