So, someone rescued or stole the remaining armour and it ended up in the hands of Kylo "Emo Kid" Ren? Wonder if upcoming movies will explain this.
Or will they turn that part of Star Wars history into a 'Star Wars story' movie like Rogue One? "A group of underground Stormtroopers stole the remains of Darth Vader's armour, but were killed one by one when delivering it to Han Solo's son." Kind of like the mirror image of Rogue One.
The one person who would have known exactly where to find Vader's helmet is Luke Skywalker. Recalling Luke's own training, a Jedi trainee is sent into a place that is "strong with the Dark Side of the Force" as a test. Luke didn't do well on that test.
The grave of a Sith Lord might well qualify as another place "strong with the Dark Side". Luke may have sent Ben Solo to Darth Vader's grave as a test, and Ben failed even more spectacularly than Luke did. For me, that would explain how Kylo Ren ended up in possession of Darth Vader's helmet.