Whether we believe that Vader should be redeemed or not is out the question - it's what the the directors intended. In this case George Lucas decided that Vader did redeem himself.
Let's look at what he does in the movie. Although he was a huge factor in eliminating the Jedi order, if it weren't for his actions at the end of ROTJ the Jedi Order could have been gone. By saving Luke Skywalker and sacrificing his life by killing emperor Palpatine, he has allowed Luke to survive, and in turn to train padawans, in turn repopulating the galaxy with Jedi.
Although noncanon, another factor to look both Yoda and Obi Wan forgave him. They would not teach him how to become a force ghost after he died if he did not consider him to be redeemed (REad The Rise and Fall of Darth Vader for more info).
It isn't a matter of Vader redeeming himself it's a matter of Anikan Skywalker regaining control. Remember that Obi-Wan states in Episode 6 that what he told Luke. "Vader betrayed and murdered your father" Obi-Wan stood by that stating to some degree it was true. Anikan was caring and loving of his friends and family as where Vader was not. Anikan brought down the Empire by killing the Emperor. If he didn't redeem himself he wouldn't be able to return as a Ghost.
One good deed is not enough to redeem a man of a lifetime of wickedness. After who knows how many other people Darth Vader killed, spontaneously deciding to save one person does not automatically atone for his other atrocities. There are also missing pieces in Vader's motives in Return of the Jedi; what motivated him to turn against Palpatine?