Anakin's fall and redemption is a theme in Star Wars, it runs through all six movies, but it is not the central theme.
Whatever Vader's status was during the Prequels, he becomes a foil for Luke, not the other way around.
Let me try and put this into a fantasy story context, maybe that'll help me to say what I want to say:
We've got the young knight on a quest to avenge his father and destroy the black knight. The young knight learns the black knight is really his father who used to be a Good Knight but fell and became corrupted and therefore the Young Knight's quest becomes one of redemption. The Young Knight is successful in his quest and the Black Knight is redeemed at the end.
That's essentially Star Wars. Just because we learn how the Black Knight became the Black Knight doesn't negate from the fact that the quest belongs to the Young Knight.
But this is brought about through Luke. Left to his own devices, Vader would never have redeemed himself.
I'll try and use X-Men 1 as an example. Cyclops is the one that ultimately shoots Magneto and takes him down, ending the threat. It was obvious that on his own, Wolverine was not going to be able to destroy Magneto's machine. But I doubt any of us would argue that Cyclops is the main character, its undoubtedly Wolverine, even though he himself is not the one that stops Magneto.
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