darklordoftech said:
Two of my headcanons about Vader:
1. People who don't know that Palpatine is a Sith Lord think that Vader is a bounty hunter hired by Tarkin. As a result, Palpatine isn't blamed for Vader's actions.
2. Vader rarely, if ever, appears in Imperial propaganda and never gives speeches on Holonet. The Tarkin novel implies that Tarkin gives the majority of the Empire's speeches.
I think a combination of the two might be the situation. However since Vader in the post-Battle of Yavin is given full control of the military, and people are commanded to treat his orders as if they were the Emperor's, it seems kind of weird to me that no one really knows who he is. He is after all "Lord Vader", and Leia in ANH seems to to be well aware of who he is. So I'm not too sure that he is so much of a scapegoat for the Emperor, but I agree that he's probably not a political figure. He's probably there to scare those who are involved in politics in check, as well as being a sort of problem solver when something threatens the Empire/Emperor.
Once the Senate becomes dissolved in ANH and the Rebel's finally prove themselves to be an actual threat to the Empire, Darth Vader get's ranked up in the military and as usual, except with much more power this time, is sent to solve a problem.
Although Vader probably never holds any speeches, he's probably a really good piece of propaganda in of himself; this mystery man that will hunt you down if you're disobedient. He's clearly feared by the Rebels, but the Imperial General's doesn't seem to take him too serious in ANH, that is of course until he proves his power. Him choking a general also proves that he, despite having little official power, can pretty much do as he pleases. And it seems likely, as you suggested, that no one knows that Palpatine is a Sith/Darksider, hence Tarkin's statement "You my friend is all that's left of their religion."
Vader may not be a general, but he seems to be Palpatine's greatest symbol/weapon of fear in his Empire.
(Maybe that's why he doesn't seem to like the Death Star, he's afraid it'll replace him.)
(PS. You mentioned "the Tarkin novel," isn't that part of the recent Disney canon?)