Luke hadn't heard of "Darth Vader" until Obi-Wan mentioned him?
Darth Vader had been Emperor Palpatine's most prominent 'enforcer', yet Luke doesn't appear to have even heard of Vader until Obi-Wan says: "A young Jedi named Darth Vader betrayed and murdered your father." When someone has been under the command of a dictatorship for decades, they are likely to know the names of the people who have power over them. I understand Owen and Beru not discussing Vader with Luke, but he surely would have heard the name tossed around by his school friends on Tatooine, particularly the ones who were planning on joining "The Academy." It's also odd that Luke never mentions to Han or Leia that Vader murdered his father in Episodes IV to VI. That's a pretty big secret to keep from your friends.
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