In Episode VII: The Force Awakens we see a scene with Kylo Ren meditating in the presence of Darth Vader's helmet.
Given that in Episode VI Luke burns Darth Vader's body on the moon of Endor, I think it would be safe to say that was Darth Vader's resting place.
So how did Kylo Ren end up with part of Darth Vader? Did he travel to the moon of Endor to specifically get a piece of Darth Vader?
This question is not how any part of Darth Vader survived Return of the Jedi but how Kylo Ren would know how to find the helmet.