I genuinely don't understand some of the posts about Vivienne plotting/poisoning The Duke. It's as though I am playing in a parallel universe. I didn't get any nefarious undertones when completing her quest. I don't see the benefit of Vivienne lying to a high approved inquisitor. The only explanation I can think of is that sabotaging the cure takes you down a different route altogether.
Maybe I am just naïve
as far as I am concerned it was a fetch quest which I delivered. It's up to Vivienne how much information to disclose about a private matter. I still treat Vivienne as a trusting companion.
if you have a low approval with her, it is very heavily implied after the dukes relatives leave that she poisoned him, she also mocks you by telling you that you carried yourself well infront of his relatives in spite of yourself, then she gleefully tells you that she used you and that you played your part well. She will tell you that now that the duke is gone, she has solidified a positive impression to the council of heralds and the chantry, of the inquisition because of your part in "helping" the duke, thus giving herself importance and weight by being included in it, and even more òf herself by taking care of the funeral and being the "good samaritan" after his death. She then goes on to say that she now has strong connections to the council of heralds (the dukes son is a member of the council) and highest levels of the chantry. She will also tell you that she is willing to do whatever it takes to get ahead in the game.
My first human mage got along very well with her and she seemed warmer and a bit less scheming, even though i always knew she had an agenda, but my canon elven mage is anti-chantry, and relatively pro-mage freedom. We didnt get along very well at all, the only thing we agreed on is that magic can be dangerous and needs to be supervised to some extent. Anyway i decided to do her quest because i wanted to do every inner circle quest, but once i got that scene with the relatives, i regretted helping the cow so much that i reloaded a save 6 hours earlier. I brought her a common wyvern heart (just to get the cutscene with her out of the way and tell her to go f herself) and then told her i wouldnt give it to her unless she told me what she intended to do with it, which she refused and ended up telling me she would find some other way to get it. Im happy to see that the quest is no longer available.
I dont care that she killed some pompous noble, but i do care that she ended up getting ahead in the "game" and that she used me to do it. Heh basically the only way you ever find out Viviennes true intentions is if she dislikes you. I think the sadness she feels when poisoning him is because she genuinely had feelings for him, but in the end, garnering power has always been the most important thing to her. To some extent she also probably justified to herself that he was going to die anyway, so she might as well hasten the process. Im thinking that the snowy wyvern heart might give him a chance to speak and make the death less painful, and maybe thats why she wanted you to get that instead of a common heart. isnt it convenient how he dies moments after drinking her potion...?