According to recent "Star Wars: The Last Jedi" spoilers, Rey sees Darth Vader and a young Luke Skywalker in her vision. This vision gives her an insight into the Jedi Master's life in the past.
According to The Express, a source at Pinewood Studios revealed that Rey's vision comes in a form of a flashback. She sees the villain Darth Vader and Luke Skywalker in a duel, reminiscent of their epic fight in "The Empire Strikes Back. "
Zeroh said that originally, Rey was supposed to see this vision in "Star Wars: The Force Awakens" when she touched Luke's lightsaber. However, this scene will reportedly now be in "Star Wars: The Last Jedi" since it is beneficial for Rey to understand Luke Skywalker's life.
This vision of Luke Skywalker and his duel with Darth Vader in Cloud City will reportedly also tell Rey how Luke lost his lightsaber. Fans may remember that the Jedi Master lost his weapon after the villain cut of his arm. The lightsaber came with Luke's hand, although the hand never surfaced in "The Force Awakens." Perhaps "Star Wars: The Last Jedi" will reveal how the lightsaber ended in Jakku and in Maz Kanata's castle all the way from Cloud City.
Interestingly, the trailer for "Star Wars: The Last Jedi" confirms Rey sees more visions of Luke Skywalker. A quick shot (at the 1:30 mark) shows a cloaked Luke put his arm out to R2D2 while they both stare at a burning building. This scene is similar to Rey's vision in "The Force Awakens" according to Movieweb.
It is likely that Rey sees more of Luke's past in the course of her Force training with him in "Star Wars: The Last Jedi." Perhaps this vision of Luke and R2D2 is a scene from the speculated slaughter of the new generation of Jedi by Kylo Ren.