I've always seen it as Darth Vader not being the most powerful man in the universe, but just a very high up guy who was close with the Emperor. Grand Moff Tarkin was in charge of the Imperial army, directly under the Emperor. When he died, Vader took over his position. In ANH Vader seemed like an enforcer type. The only force user left, working for the Imperium. Like, Tarkin was in charge, but Vader actually went down on "away missions". I know it's sacrilege to talk about Star Trek, but I'm thinking like Picard and Riker. Picard is captain of the ship, but Riker is directly under him and is the guy who actually gets his hands dirty.