I've just obtained the "Grateful Vess", and I feel like shit for unlocking it.
I took the evilest route with her since chapter 1, by not telling her about the plan. And it kept going on, by choosing the mean replies and never caring about her. Not kissed her, not talked to her, and treated terribly during her jail time. I've also tried to leave her there.
Then, in chapter 13 I had the choice to ignore her again or to unlock the scene.