Am I the only one who *didn't* interpret this as poison? The potion was called an "age regression" potion. I thought she was trying to save him and it just didn't work? If she just wants to give him a poison to make him fall asleep and die, she doesn't need especially exotic ingredients for that.
I didn't interpret it as poison on my first playthrough. I took Vivienne at her word, as despite my personal dislike for the character, my Inquisitor had a sense of respect for her.
On my second playthrough, playing a dalish mage this time, not so much. Did the quest, but when Bastien's family came to Skyhold I decided to take the hardline path of accusing her of using me. She gleefully admitted to it and seemed down right jolly that Bastien's death gave her access to the grand clerics and council of heralds.
So I think poison is a perfectly valid interpretation of the scene and Vivienne's character.