Knights of the Eternal Throne made me feel bad for Vaylin, it was hinted at several times during the first expansion Valkorian did something to her, but we never learned what he did until we followed Vaylin to Nathema, at that point my opinion of Vaylins character changed.
She was abused as a child by Valkorian, conditioned into becoming what she was, Vaylin had more depth to her character than most of the main companion. (Arcann, Lana, Theron) It's a pity they ended it the way they did, I would have enjoyed learning more about her character.
My playthroughs of the Knight expansion packs was as a Jedi Counsellors, I've been healing Sith from the start and it never occurred to my character to attempt such an act on Vaylin? or Arcann?
I don't know if redemption would have worked for Vaylin, but the option to try would have been appreciated, Vaylin deserved that redemption plot a lot more than Arcann.
Can they bring her back? Sure! This is Star Wars and the possibilities are endless, especially if they're doing a Star Trek III plot to Nathema.