A reply to this ^ and "He bounces back between being one of the 13/an Ancient Darkness and Ventus’s own."
I personally believe he is both. In subtitles when he referenced to himself in KH3 as darkness, the D was always capitalized, referencing a name or person, not just the general darkness. He showed up to shit talk the Guardians with the Norts, probably showing that Xehanort knows Vanitas means shit.
As for BBS references, I can't remember any atm.
And then we have Dark Inferno X, either being a form/connection to Vanitas, I believe Vanitas pulled a Sora and jumped to Ventus and was talking to Sora as his pure 13 Darkness self.
This is just me just small brain theory-vomiting.