Actually I think that Vader's "rank" in the Empire was most similar to Vice President of the United States. The Vice President has almost *no power* given to him by the Constitution: all he does is preside over the Senate (and he can't even vote unless there's a deadlock; it's such a boring job, most VP's leave it to the pro tempore) and wait for the President to kick the bucket. He only has the power that the President decides to give to him. Vader's a lot like that. He's outside the chain of command, but Palpy can give him whatever power he sees fit. And of course, he has the prestige of being Palp's right hand man.
As others have said, the Death Star was Tarkin's turf. Because of his contributions to the Empire, Tarkin reported to Palpy alone. Vader was probably told to defer to Tarkin's decisions... unless Tarkin got too big for his britches and had to be taught just who ruled the galaxy.
Is it a crime to try and learn the truth? Is it a sin to search for those things which you fear? My purpose in this world is knowledge, and the dissemination of it. And it is I who is to restore the fruits of my labors to the entire world. Wake up! Don’t be afraid of knowledge! Humans who loose the capacity to think become creatures whose existence has no value. Think, you humans who are split into two worlds! Unless you want the gulf between humans to expand into oblivion,YOU MUST THINK!- Schwarzwald
On the DS, Vader's orders were probably to defer to Tarkin unless Tarkin was committing treason. Tarkin probably knew it, too; no one in his right mind says "spare him" when Vader's pissed unless the Emperor says he can.
Fragment of the Lord of Nightmares, release thy heavenly retribution. Blade of cold, black nothingness: become my power, become my body. Together, let us walk the path of destruction and smash even the souls of the Gods! RAGNA BLADE!
Lore Monkey | the Pichu-master™
Secularism—since AD 80
Av: Elika; Prince of Persia
