She— It?— is wrong. Vanitas knows he'll be this half a person for eternity, without Ventus, without Sora. Even with the x-Blade in his possession, there's no relief to be found in it. Without the other half, it's only a weapon, like Vanitas himself. But with the adrenaline pooled out of him now, held by this dark monster with it's low, stilted voice, and the weight of the entity's presence, he's too tired to keep fighting. Whether the spirit will stay or not, just like that, all the strength goes out of him and Vanitas passes out.