Why is Darth Vader trying to overthrow the Emperor in this game?

He's a Sith. That's what they do.

Here's my big question about this game- how the heck could Bail Organa be a free man and on Alderaan when it was destroyed according to numerous other sources when he'd already been outed as a member of the Rebellion, captured by the Empire, and TAKEN ABOARD THE PARTIALLY COMPLETED TOP-SECRET DEATH STAR??? Is there any way he wouldn't have to be in hiding for the rest of his life? The whole point traditionally has been that the Organas were secret supporters of the Rebellion, not known, identified, and captured fugitives that were then allowed to return to ruling a sovereign planet. (Of course, Bail Organa was always established as the ruler, too, so clearly the continuity is already bent, but I've never heard any source suggesting that Bail wasn't on Alderaan, or ruler himself, by the time of ANH, and certainly nothing suggesting he was an Imperial fugitive.)