When outlining ROTJ Kasdan pitched the idea to Lucas to have Luke put Vader's helmet on as a feigned show of allegiance to the Emperor. Helmeted Luke was then going to request that the Emperor allow him be the one to press the button to have the Death Star blow up the Rebel Fleet but instead he changes the target and destroys the Emperor's homeworld instead (which I believe was called Had Abbadon at the time.) So there *is* a precedent for a hero wearing Vader's costume, even if it was a very early nugget of an idea that was veto'd by Lucas.
I really kind of want to see Darth Rey now.
Actually in all seriousness I could see it playing out this way:
Rey dons Vader's costume. Pretends to be Vader, approaches Kylo. Kylo lets down his guard and reaches out to his beloved grandfather. Rey runs the son of a ***** through with a lightsaber. Kylo's last few seconds of life are spent in anguish as he believes he is being murdered by the one he loves the most. Poetic justice for what he did to his dad.
Maybe even structure it as a way so that's it's a bit of a surprise to the audience. Like we see that Kylo has a case with a newly restored Vader costume. Later Vader appears to him and we assume that he was somehow resurrected but then when he kills Kylo "he" removes his mask so that we see that it was just Rey who stumbled upon the outfit.
