Originally posted by: ScruffyOriginally posted by: Marvolo
Tarkin was the Commanding Officer of the Death Star. He had the final say of anything that involoved the Death Star, unless the Emperor gave an order. So when Vader was on the Death Star he was out-ranked by Tarkin. However, when Darth Vader was not on the Death Star he answered only to the Emperor, and no one else.
The relationship between Vader and Palpatine does not confer any particular rank or authority to Vader. If Vader is subordinate to Tarkin on the Death Star because the Death Star belongs to Tarkin, he would also be subordinate to any other Grand Moff/Regional Governor when in that GM/RG's territory. This legal reality is probably why Vader was installed in Death Squadron and given a mandate to operate freely throughout the Empire's regions; after Vader's disastrous losses of the Death Star plans[1] and generally bad behavior, no Grand Moff would dare let him embark on one of their ships.
[1] He lost them, got them back, then lost them
again.