Was he like "who the heck is that guy in black and why did he just cut old Ben in half?" When Ben is dropping "a young Jedi named Darth Vader" story on him, does Luke even know about Vader as a person? Did Imperial citizens in general know about Vader? Would Luke really catch up on the exploits of the Emperor's top dog enforcer living on a sandball like Tatooine? (Luke did want to run off and be a pilot for the Empire but how much does that translate into, like, Luke having Darth Vader posters on his wall at his Aunt and Uncle's house?) Or was there an awkward conversation at the checkerboard on the Falcon between Luke and Leia where Luke is all like "that guy in the weird cape and helmet kinda murdered Ben." and she's all like, "Yeah, uh, that guy is named Darth Vader." and then Luke is all like "Oooohhhhh! THAT'S the guy who Ben told me about. Who betrayed and murdered my father, too. What a jerk!"