I have unlocked it yesterday. You MUST treat her as worst as possible since chapter 1. Or at least it's what I made, and in chapter 13 I got the scene.
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agree'd. vess is a good girl in a bad situation (a girl that is good hearted - totally unrealistic, but hey, thats her character and ill take it at face value). no reason to treat her bad at any point in the game. the only ones that are worth treating badly are those committing fraud after a bit of genital mutilation like Raene and Ziv. and if i remember right, Ziv was actually bortnm that way, being the alien that she is. but Raene is literally mentally ill, wish i could just hand him back to his father, or the nearest hospital. he needs help i am not qualified to give.