Vivienne's ambition clouds her judgement. She does not make choices based on the morality of something, but on the best possible outcome for HER. That is not to say she has no morality, that is to say she believes her morality should shape the outcome. This can be observed if she is the Divine; a tyrant. She needs as little power as possible, she reminds me of a politician. lol
She's not a tyrant. People try to over throw her as Divine, and she stops them, each time. No different from how Justinia used Leliana to assassinate people she thought needed assassinating.
If the mages were conscripted, they try to start ANOTHER rebellion which she quickly puts down. Under Vivienne, mages have more freedom and responsibilities. Kind of makes the second attempt at rebellion a bit moot.
The only thing she does that is a bit like a tyrant is keep the templars under her direct control - I imagine to try avoid the fiasco that was Kirkwall, and you know, the whole red lyrium thing. Considering Cass will be rebuilding the seekers who are supposed to be top of the templar order though, and that Cass and Vivi get along quiet well I don't really forsee it being too much of a problem as of now.