Why didn't Darth Vader recognise R2D2 when he boarded Princess Leia's shuttle at the start of A New Hope?
That's a good one. My best guess is it's a continuity error. I'm sure a canonical explanation has been created however. Vader may have recognized him, and just not cared because he was so consumed by the darkside. Also, R2 is a particular model, so it stands to reason that he's not the only blue and white R2 unit in the galaxy.