I do not like Vivienne because she is only in it for herself, a quality that I find distasteful in people in games as much as I do in real life. Everything that Vivienne does is ultimately to advance her position and interests. She seems to lack compassion and empathy. Even when her lover dies, from a weird concoction that no one has ever heard before in game that somehow mysteriously failed, she shows little sadness and instead uses his death to get herself more power.
Some might say that the fact that she warms up to some of the followers as well as the Inquisitor and use this as proof of her being redeemable. However, does it really say much about someone that they have the ability to make friends? Even self-absorbed jerks want to have friends, because it gives them people to surround themselves with who will reinforce how awesome they are.
You know that saying about "If you on a date and your partner is nice to you but mean to the waitress, they are not a nice person"? That is Vivienne in a nutshell. She simply has no regard for people outside of her circle that do not have something to offer her. Look at how she treat Blackwall, for instance.
It's the same "F**k you if you're not my friend" attitude that you see in politicians and other rich powerful people. If she tithes to charity, it isn't because she cares about the poor and the weak, it is because it is the politically correct and expected thing to do. It is no different when some politician poses with some cute babies for votes.
It has been mentioned and it is common knowledge that Vivienne is supportive of Templars but not Mages, but did not touch on the reason for that: See, supporting the Templar order is a politically correct thing to do with the people Vivienne wishes to ingratiate herself with, the nobility and the Chantry, while supporting mages is not. Therefore, Vivienne does not support the Mages, even though she has had every opportunity to understand the motivation behind the mage rebellion.
I can imagine Vivienne being very pro-Mage if she was born and raised in Tevinter, because that is the politically correct opinion there and because being pro-Mage in Tevinter would let her move up through the Magisterium and the Imperial Chantry.
Does this mean I hate Templars ? No. I can see myself getting along well enough with Cassandra and Cullen and their ideas, especially Cullen's idea on how Mages should be supervised and Cassandra on how she plans to reform the Seekers. However Cullen and Cassandra have these opinions because of something more profound that political correctness.
As a player, I really cannot fault the writing because there are plenty of people like Vivienne around in the real world, and people like that would inevitably try to play nice with the Inquisition in order to advance their goals. For what she is, she's written quite well.
Nonetheless, from a roleplaying perspective I think she is a shallow and self-absorbed person who I know would turn on anyone in the party or the organization if she thought it would offer her a leg up in the grand scheme of things. I would go as far as to say that Vivienne reminds me of Littlefinger from Game of Thrones and no, that is not a compliment.
In that respect she is completely untrustworthy as a companion and a threat to the integrity of the Inquisition if she decides that throwing her lot in with another organization or another group would serve her better. I keep her at arm's length to keep an eye on her, but I would never trust her to have my back and I would not give her the opportunity to acquire any power.
Tl;dr: Scheming politicians suck.