Or he may have felt a disturbance in the force when he was trying to shoot Luke down and could feel that there was some relation. Just a thought. We know he felt a disturbance in the Force. "The Force is strong with this one." The Marvel Comics had a great storyline on Vader's hunt for the name of he pilot who shot down the Death Star. The late, great Archie Goodwin wrote it. Anyway, despite that wonderful story in something that most people won't admit into the EU, I think it went something like this. Vader didn't know what his troops did on Tatooine. The tapes came to Alderaan, after all, so why dig into the reports. But after spending a few months in a TIE fighter limping home from Yavin, he backtracked it. The ship came to Alderaan from Tatooine. Kenobi was on it, but someone else with Jedi Skills was also on it, or that tremor in the Force wouldn't have been there. So. Hmm. Troops track the pod. They shoot some Tusken Raiders to get the banthas. he gets wistful for a moment, thinking of the good old days when he was young and slaughtered Sandpeople unmercifully. Sigh. then he grabs another hot chocolate, a passion he shares with his son in the EU, and keeps reading reports. Oh yeah. Someone is going to have to teach those troopers how to spell Jawa. It's a double-u, not v. Two droids. An R2 unit and an old fashioned protocol droid. Hey, I used to have droids like that. Really handy, too. Oh, they got them from Lars. That's where Mom is buried. Oh, they burned the place down and killed my step-brother. Surly cuss. Oh. they killed his wife too. She made a great YOGURT. And the blue milk was just the BEST. Damn shame that. Oh, Beru said her nephew was missing. Who's that? Duke Skywalker? Kid has my last name. My last name. And by backtracking, the pieces fall into place. Vader can know nothing, conceivably, during Star Wars, and be searching for Luke by name at the time of Empire, no problemo.