I think Darkness is Vanitas but only to a certain extent. I don’t think they’re gonna retcon Darkness into not being the darkness inside Ventus’s heart, but rather Vanitas is a combination of the darkness in Ven’s heart and the Darkness being residing inside of him. Hence his boyish personality compared to how we see Darkness in Union Cross.
Will we see Vanitas again? Maybe. But what we will definitely see is the reoccurrence of Darkness and it’s fellow beings since it seems they’re being set up as the main enemies of the Master of Masters. One great thing about Dark Road is that each episode is giving insight into what Darkness is, how it comes about, and how it works. It seems like Nomura is using the app to give us insights into the future workings of darkness in the series.
My guess is that he’ll use the beings of Darkness in tandem with Maleficent and Pete to make them an actual force to be reckoned with. They’ve been doing nothing for so long that I know it’s hard for people to believe they’ll ever do anything, but I sincerely think that Nomura has something in mind for them. Otherwise, he wouldn’t have kept them around.