I confess I don't understand some of the reactions here. Vivienne does join so that she can increase her influence (at least in large part).
First, how does that surprise anyone, she's marketed as The Ambition and is clearly a person who thinks ahead and looks out for number 1.
Second, so what? What does her reasoning for joining have to do with anything? She's still out there fighting Corypheus like anyone else in the party, that's all one should care about I figure. She has as much influence as you give her, in the end.
Third, it's not like she's all talk. Her ending as Divine clearly states that, while she concentrates power in her hands, the Circles are more free than they used to be. She does work for the betterment of mages; the divide seems to be what one considers betterment, but it doesn't make her someone who only cares about herself.
And finally, next to the blindingly stupid Fiona and her crew, she kinda comes across as a viable alternative to me, even with her numerous flaws.
In response from my view point..
1. Surprising is a bit of a stretch considering if you read between the lines of how Vivienne acts and speaks when you first encounter her. There is nothing wrong with having ambition, hell everyone within the Inquisition can be held accountable on that front. Vivienne however takes her ambitious nature and increases it twofold. Here is someone who not only plays but enjoys The Game. Anyone who enjoys such devious actions is one not to be trusted. Perhaps it is just who I am as a person, I tend to think of others before myself and Vivienne violates my compassionate nature thus making me despise her.
2. Overly ambitious people will do something not because they want too but because they have too. Understand, Vivienne joins the Inquisition not because she wants to stop Cory but because she wants to use The Inquisitor and The Inquisition to further her own career.I am sure some where down the list of her priorities stopping Cory is there, but it is not the main goal. A villain can stop to fed a puppy but one act of compassion does not mean I should over look all they wrong he/she has done.
3. This goes well beyond her view point of mages. By all means Vivienne can have her own views on mages. I may not agree with them but not everyone can agree on everything. A person like Vivienne becoming Divine is a scarey thought. Give an overly ambitious person power and everything MAY seem fine at first, but sooner or later they will want more power. I have a feeling that down the road making Vivienne Divine will end up causing chaos upon chaos. Considering how Vivienne is, making her Divine is no better then just handing the Sunburst Throne over to a Tevinter. Vivienne reminds me of someone from real life history, but I shall not name that person here or ever.
Finally...
As for Fiona, yes she did make mistakes, I for one am pro mage yet I do not agree with Fiona's actions. I do however have more respect for Fiona then for Vivienne. Fiona did all she could to safeguard her fellow mages, she cares so much that causes Fiona to make bad choices. I admire Fiona's compassion and her selfless nature.
Vivienne on the other hand cares only for herself and will care about others only if there is something she needs from them. Oh sure, Vivienne puts on a grand show with The Duke, but anyone as manipulative as Vivienne can do the same. I do not believe for a second that Vivienne truly loved this man, his money and status, but not the man. But I digress, Vivienne is a selfish person not the sort of person I could ever idolize much less give two bits about.
In my eyes Vivienne is the villain within The Inquisition walls, a snake in the grass. I no problems with ambitious people in or out of the game, but when someone like Vivienne takes it as far as some politicians do, that is cause for concerned. I'd be far more comfortable with an assassin hiding in the shadows then be in the presence of someone like Vivienne.
Out of all the companions Vivienne is an odd fit. She belongs in Orlais throwing parties and proving whatever it is she needs to prove.
Compared to the other companions the only thing she brings to the Inquisition is giving me a headache. Thus I believe from this point on, I will no longer recruit her. Sure this may seem strange to some but I've already played a game without Vivienne being recruited and to be honest, it was nice not having a Viper in the grass.