Darth Vader did not deserve redemption.

On Sideous and the Empire - Vader still had a choice at many differing intervals to kill Palpatine. On Special time we need to remember is the first and potentially most important time he could have done so in Revenge of the Sith. Master Windu had Palpatine in his grasps, and all the acts of the Empire could have instantly be stopped. Anakin at that time had full knowledge of who Palpatine really was, what he was capable of doing, and to a point what he had done. Palpatine's manipulation is not a factor, since Anakin always had a choice, something Sideous knew full well. Anakin could have assisted Windu and ended Palpatine, but chose to kill the Jedi rather than the Sith. Instead, he delayed the deed he should have done for years until he felt pity for his son. He didn't even kill Palpatine for realization of what he'd done throughout the Galaxy, but for selfish reasons AGAIN! Had it been any other person, Palpatine would have electrocuted the victim, and Vader would have stood there and watched, as he probably had done hundreds of time's before. But even when he killed Palpatine, it wasn't him who had won the Battle of Endor but the Rebels. Han and Liea had destroyed the shield Generator. Lando had already started flying into the Death Star. It would have blown up either way regardless of Palpatines 'death' in a trench or not. The Empire had lost the Executor, the Death Star, and may Star Destroyers, not to mention the personnel on those craft and planet side on Endor. They were crippled anyway. Even if Palpatine had made it off it would have been difficult to pick up the pieces from such a major defeat. All the events taking place afterwards would still occur. The Eclipse would still be constructed, and the Empire would still have survived. So still, Vader's actions did nothing to stop the further and current strife of the Galaxy. In fact, he had the possibility of stopping it all years before, but didn't, making the case against him even worse! The Jedi would have lived if Palpatine had died in his office, as Order 66 wasn't instigated until after Anakin saved him. The Republic was beating the Confederacy (I say begrudgingly), and was overall close to winning the war for the Republic. But Anakin ruined all of that. It wasn't Palpatine who made the Empire, but Vader. He's even more responsible for his actions, which brings me to your next point. On Jedi - Once Anakin swore allegiance to Sideous and became Darth Vader he lost his Jedi-hood. He was now a Sith. He was one of the 10 Betrayers, including Dooku, Exar-Kun, and others. He had no more right to re-become a Jedi than any of them did. He would have been banished from the Jedi Order and sentenced to exile or prison even if he had repented under normal circumstances. He also was not the savior of the Republic, but its destroyer. Like I said above, he is the true forger of the Empire. It was never the same after Palpatine took it over. When the New Republic began it never really regained all the prestige it had once had as the New Republic, and in the end just became the Empire again 125 years later. Vader is responsible for all of that. For him to have truly been able to redeem himself from all that he would have needed to do something that would definitely restore the Republic and Jedi Order, and only the Force knows what that could have been. His redeeming action accomplished neither of these things, and could be argued that he made it worse. He did 1 good thing, but that came no where close to making up for the trillions of horrors he caused. the Force still should not have accepted him.