✿.

Vanitas certainly doesn't have much agency in how he lives his life as a child, as no kid gets to pick their parents or circumstances, and Vani has it especially rough with the death of his mother hanging over his head. Then his father dies protecting him, and he dies in a situation in which Vanitas is utterly powerless. Horror erupts into his life out of nowhere, and his dad throws himself in front of some fangs, and there's nothing Vanitas can do but watch it happen.

Next there's Vanitas's brief time training as a chasseur, which is one part of his history we know little about, so it's hard for me to say how free his choice was. He may have been pressured into joining, as we know the Chassuers aren't above pushy/manipulative recruitment of children (Astolfo), but I could also see his choice to hunt vampires made as a trauma response to the powerlessness of the vampire attack he survived. "I was powerless, so I'll claim the power to enact violence and make sure they can't hurt me or others again."

Then Vanitas is abducted by Doctor Moreau, which strips him of his agency just about as awfully as anything possibly could.

Vanitas the test subject has no bodily autonomy. He is poked and prodded and experimented on, because his body is an object of science to Moreau, not the vessel of a person with rights or self-determination. His only big active choices during his captivity are the choice to not run away for fear of somebody else suffering and the choice to volunteer in Mikhail's place. The only choices he can make are the choices to stay and throw himself even deeper into his own violation. Even his relationship with Misha is something that just kind of Happens to him. The kid is a force of nature that Moreau dumps on him without his say.

(This isn't Mikhail's fault, as Misha is just a child that wants affection, but having a needy, vulnerable little brother figure suddenly dropped on him in the midst of all that horror couldn't have made Vanitas feel less out of control).

Then Vanitas's torment by Moreau hits its climax, and Vanitas is told that not only has his body been violated by way of pain and torture—even his basic humanity has been and will be stripped from him. He now feels the disgust of having the blood of something he hates inside of him without his consent. And he's about to be killed, turned into a mindless husk of a "living key" instead of a person.

And that's when Luna shows up. This mysterious, incredibly powerful being appears and wreaks havoc on Moreau's lab like an avenging angel, and it agrees to Mikhail's request to take "us" along with it. Everything in Vanitas's life so far has taught him that he is powerless in the face of others' great power. Horrible shit just keeps happening to him forever, and this monstrous person sounds like they've just agreed to carry him off on Mikhail's request without a word of input from him. Of course he doesn't expect to be given a choice.

But he is.

Luna scoops up Mikhail, then they turn to Vanitas and ask if he would like to come along with them as well. After months or years of absolutely all of his autonomy and power being stripped away from him, the most powerful being Vanitas has ever encountered stops and gives him a choice about what he'd like to do next. They give him the option to go off on his own and decide his own fate rather than go along with them. And that's why Vanitas doesn't run away.

These are the images that flash through Vanitas's head right before he gets up and runs to Luna:

These moments are what inform Vanitas's choice. The memories of his father's death and Moreau's final experiment on him—his two most awful moments of abject helplessness.

When Vanitas runs to Luna and accepts their offer, these moments are what he's running from. He's fleeing from his inability to do anything or control his fate. He's running from a life of stolen agency. He's running toward the one adult who actually stopped and gave him a choice about what he'd like to do next.

Vanitas chooses Luna because they, despite having the power to kill or subdue him, give him the freedom to choose to accompany them in the first place. He's drawn to them not for protection, but as an escape to a life where he gets to keep making choices and grasp some agency.

Luna is the savior that gives Vanitas his freedom and autonomy back after it's stolen by Moreau. That's why it's so tragic that Vanitas carries their Mark in the present day. It's undeniable proof that, in their right mind or not, some version of Luna violated the fundamental trust and gift of autonomy that brought Vanitas into their family in the first place.