)"But who's to say Vader wouldn't have taken out Tarkin?"
Like he did with Palpatine? You know, the guy who was more powerful to begin with? Sorry, but your logic is becoming circular. Vader followed Tarkin's orders, and reported to him several times throughout ANH. You are more than welcome to read more into the all you want, but Vader is clearly under the command of a mere human.
"Tarkin is able to say the things he does because he knows the Emperor has his back."
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In fact, [Vader] is one of the Dark Lords who is working for the Emperor, and he has to do the Emperor's bidding. You will see at some point in the future that the Jedi have the same relationship to the Republic, they're likepublic servants, they're like marshals or policemen. They basically do what they're told to do. They're not independent agents who can do whatever they want. In the case of the Jedi, they work for the Republic, and in the case of the Sith Lords, they work for the Empire. So I obviously didn't want to make Vader too weak, but I didn't want to make him so you thought he was in charge of everything. That's why I had Tarkin in the first film, although he was more of a bureaucrat. - Annotated Screenplays