Since 2014 the Darth Vader vs. Luke Skywalker on Bespin sequence being reshot for Star Wars: The Force Awakens has been something of discussion amongst sources with various levels of confirmation. The sequence was to happen when Rey enters the “Forceback” or flash back sequence at Maz’s Castle. Over the course of the production we heard rumors that there was an actor cast in the role as “Young Original Warrior.” The actor as Luke is still is not confirmed and neither is the person who played Darth Vader. In essence, we see Rey watch the fight from a new vantage point. It seems as if the Bespin pieces we see in the film were to be part of the set from which Rey watched the father and son duel.

A new book has hit today called Star Wars The Force Awakens: Rey’s Story: 

On pages 99-100 of the book we have the flashback reiterated with the missing sequence described here:

As she peered into the distance, she saw a man in black wielding a red Lightsaber. His face was covered with a mask, molded to approximate a human face. But the mask’s eyes were dead and empty. The man’s red Lightsaber clashed against a brilliant beam of blue. A young man with blond hair raised his own Lightsaber and fought furiously against the monster in black.

It also calls the the man killed by Kylo Ren in the flash back a hero:

Before she knew it, more images flashed through her mind: a hero stabbed by a fiery blade, a battlefield filled with soldiers, a blue and- white droid beside a lost Jedi.

I hope it shows up on The Force Awakens Blu-ray. I highly recommend picking up Rey’s Story as it confirms a lot more than you’d think.