Vanitas never says the word “darkness” in BBS. Let’s talk about why.
Up until Ventus and Vanitas’ final conversation in Kingdom Hearts 3, Vanitas never mentions darkness once. Surprising, right? He never mocks Ventus for not using darkness, he never goes on monologues about how the darkness is great and how you shouldn’t fear it…
This makes Vanitas unique among KH villains. If anything, Vanitas might be one of the few characters who knows that the idea of power can be separated from darkness. This is because he needs Ventus, pure light-hearted Ventus, to get stronger without using darkness in order to forge the x-blade.
(Well, he is using darkness, but it’s Vanitas supplying it—pushing Ventus further with the Unversed…)
Vanitas only talks about darkness when other characters bring up the subject. Xehanort mentions Darkness in Dark Road and Vanitas is all dismissive, saying "Hmph. It has nothing to do with me" and "Who knows?”.
When Xehanort suggests Vanitas fearing the light, Vanitas’ mind goes to Ventus. Not the essence of light. Ventus. To him, the idea of light is inextricably linked to his other half, perhaps inseparable.
The only reason Vanitas talks about darkness in Kingdom Hearts 3 is because Sora brings it up:
Sora: "Then why won't you stand by our side? Instead of with darkness?"
Sora’s thinking of the big picture: the heroes of light versus the villains of darkness. He doesn’t realize just how deeply personal Vanitas is driven.
Vanitas has always thought of himself in relation to other people, never just to pure darkness. In Dark Road,
Xehanort: "Darkness was once whole. And though fractured since, their minds were once connected. You must know where the others are.”
Vanitas: “I'm my own being.”
Xehanort: "That was once part of a whole?”
Vanitas remains silent. It's almost as if he’s too annoyed to answer, as Xehanort somewhat dials back the pressure in his next question.
However, in Kingdom Hearts 3, Vanitas says,
"I'm the piece of Ventus that was taken away. And you're the piece Ventus needed to be whole again… You define me, Sora, the same way that Ventus does. We are brothers who, together, make a greater whole.”
He chooses to identify himself through Sora and Ventus instead of Darkness. They are whom he feels a sense of wholeness with.
So, yeah, when Vanitas says “I AM darkness”, it’s hard for me to believe he’s actively cheering on the forces of darkness in the same way Xehanort does. Perhaps, Vanitas sees darkness and light as a set pair. If his concept of light is tied to Ventus, his own self might be tied to darkness in the same way.
