glass said:
Interesting question. How much did Tarkin know about Vader's relationship with the Emporer? It seemed to be quite strongly implied in the original trilogy that noone (except Vader) new the Emporer was a Sith lord.
Judging by RotS, Yoda and Obi-Wan knew as well, but they were hardly in any position to shoult it from the roof tops, and the emporer and vader between them silenced the other possible witnesses.
Given how both high Imperial brass (General Tagge) and low Fringe scum (Han Solo) disbelieved in the Force, and how only the Rebels who carried the torch from the old Republic seemed to believe in the Jedi, it seems pretty clear that the Emperor wanted to quash all belief in the Force. The Jedi were to be thought of as crazy old wizards, crackpot mystics, fanatical terrorists who tried to assassinate the beloved leader. The Sith were just some odd term for something to do with the Jedi, somehow, that apparently the Emperor didn't object to, maybe they were the Jedi who sided with him, but only Vader survived (average galactic citizen's opinion).
It seemed pretty clear that to all but probably the surviving Jedi (Palpatine's true nature was probably announced on that Beacon that Obi-Wan set on Coruscant) and other outcasts and enemies of the state, the Emperor was just a beloved and powerful leader who protected the Republic through his bold leadership in the Clone Wars, survived assassination attempts by the fanatical Jedi, and rebuilt the smoking ashes of the galaxy into a New Order without the impediments of earlier ages, or "throwbacks" like Jedi.
Vader being an open Force user and not hiding his beliefs was seen as quaint ("don't frighten us with your sorcerer's ways, Lord Vader), and tolerated because he apparently was the Emperor's Right Hand Man (after all, he saved the Emperor from the Jedi assassination attempt, making him the only Jedi who was "loyal"). Of course, the highest levels of Imperial command learned to fear him (or else), and probably the grunts were superstitious and there were all kinds of rumors about what he could do (probably mostly true), but to most of the Empire he was just an interesting throwback to an earlier age, kept around because of his absolute loyalty to the Emperor.