In the new Disney/Marvel canon, in which all new comics, books, and games are considered to have the same level of canon as the movies, Vader seemingly figures it out after a confrontation with Luke, in the monthly comic series Star Wars issue #2. There's a preview of it here, but the relevant part happens right after the preview ends, where Vader takes a closer look at the lightsaber and realizes it used to be his own. I haven't read the full issue, so I'm not completely sure what happens at this point, but this is likely to be the only currently-canon source for Vader figuring it out.