As I said in my article about Luke Skywalker, I do't think that Vader would've chosen to turn to the Light Side again on his own. He was VERY VERY hesitant and disbelieving when Luke first told him of his former life and all the good he used to hold in his heart. Also, why would he need to turn to the Light Side again on his own when things were working out so perfectly for the Empire? At the end of Episode VI, Sidious and Vader were positive that the Rebel Alliance was about to fall, so I highly doubt that Vader would've considered leaving the Empire and joining the good guys again when it seemed like the Alliance was about to fall through the cracks.

I think he wouldn't have turned back on his own. Luke's love for him was the only thing that did it.

He had nothing to live for anymore, he had lost everything.
Until he found his son. And that love was enough to turn him again.