Okay, some of what I will say has already been said by others (but I'll just repeat it anyway
).
Firstly, no-one should be shocked about Vivienne's opinions because she tells you her Circle stance from the outset. She gives us a pro-circle point of view as a mage and she does this by giving a logical, rational argument. I like the fact that she is resolute in her beliefs and is not influenced by the inquisitor/anyone else.
Vivienne acknowledges that there are abuses of power with the current system but it is better to fix the Circle system rather than create new problems with a completely new system. Vivienne is a character that supports the mages and sees the worth of having Templars around (I think she refers to Templars as a tools to be used). So she isn't a massive fan of the Templars but she can see they're a necessity and have an important role against magical dangers. I don't think Vivienne is absolutely against mage freedom but it should be a gradual change rather than instantaneous.
Vivienne wasn't awarded power overnight. She worked extremely hard by playing 'The Game' and playing this game as an outsider (Non-Orlesian). She is ambitious but this can be viewed as a positive because it shows she works hard and is motivated to achieve her goals.
When speaking to Vivienne about becoming Divine, I don't feel that she is trying to manipulate you in choosing her. In fact, my inquisitor suggested her name before she even put her hat in the ring. I also don't believe that Vivienne had a secret ambition to be Divine. Vivienne accepts the inquisitor's recommendation because there is no suitable alternative.
If she is Divine, she grants more freedoms to the mages (but we don't know how far this freedom extends) so she does care about her fellow mages. If she was just looking out for herself then she could have just reinstated the Circles, then sit on the throne and done nothing.
Overall I think Vivienne as a Divine could work. She restores the institutions of the Circle, Templars and Chantry and she is familiar on how they operate so she knows what needs improving. I believe the Circle system can work and it is useful in training mages. This system needs rational and sane people on both the mages and Templars side to prevent incidents like Kirkwall. The Circles must have been doing something right to survive for centuries. Vivienne has a strong influence on the Templars and this can be viewed as a good thing. The Templar reputation among the faithful will be at an all time low because they have just rebelled against the Chantry. So Vivienne will need to be seen as being in control to restore this lost confidence. She has them on a tight leash so if there is a faulty Templar 'tool' then she will replace it. She also has experience of being a mage in the Circle and will be the first Divine what has had first hand experience of the system. Vivienne as a Divine could become an inspiration to the mages and the faithful (because she would be a bridging connection between the two) and this consequently could change people's view on mages as a whole. If public perception of mages is more favourable then Vivienne in theory can grant more freedoms in the future.
Vivienne can come across as self-serving and arrogant and this is due to the Orlesian culture. She needs to keep up appearances in order to not look weak and maintain her reputation. However, her true inner feelings are sometimes shown and it shows a softer side. For example, Vivienne is the only companion to ask the inquisitor how they were feeling after Adamant and the Well. She also gives approval if you save the people at Haven so she does show concern about other's feelings. Vivienne could have easily sat back in the palace but decided to join the inquisitor to be proactive and fight for what she believes in.
I saw a post about what would she do for non-human believers to discourage discrimination. You could make this as an argument against Justinia or the Chantry as an organisation as a whole. This would not be Vivienne's problem until after she is made Divine. If Vivienne does become Divine, it would be just speculation at this point on what would happen but who's to say that she won't grant further freedoms like she did with the mages.
Furthermore, mages wouldn't be locked away in the Circle. Vivienne says that permission from the First Enchanter is needed to leave the Circle. I don't know how often this permission was granted but the important fact is that such a permission exists. Therefore, Vivienne isn't a hypocrite because she's taking advantage of existing rules.
The most convincing argument against Vivienne would be disliking her due to her personality or she is too conservative in her reforms (although this is still speculation as the reforms weren't given in detail).