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?