As strange as it sounds, I like Vivienne as a character because I don't like her.
If I were to meet someone like her in real life (and I have), I wouldn't be able to stand her.
But I recruit her anyway, because from a purely practical standpoint, why would I let someone like her operate unchecked? By having her close by, I can more easily keep an eye on her, as well as get valuable input into all the "behind the scenes" stuff.
In addition, I can keep her in check by quietly "non"-supporting her bid to be Divine.
Even before I looked at the epilogue, I knew that if Vivienne became Divine, then it would only be bad news. Every time she would bring it up, I would act "simple" to make her feel like she was in charge, but then the corresponding War Table operation that would come up would be easier to handle, since I at least know what "NOT" to do (i.e. anything that would give her more power, direct or otherwise).
And as tempting as it is to give her the fake heart and rub it in her face later, to do that would make me no better, and I have no reason to give her concrete evidence to be actively hostile towards me.
In a nutshell, I don't need to be heartless (not a pun, more... poetic).
Also, she does have some good points. She's right about magic, it's powerful, and those who wield it need to use it responsibly.
In combat, she's very powerful, what with my favorite specialization and all. (Knight Enchanter for the win!)
To put it all together:
She's very smart, and very powerful
She knows how to handle the more disgusting parts of running a "global" organization
She DOES have the right idea on a few points
I'd rather have someone like that on my side, rather than risk having her become an enemy later
AND...
She's not all bad. She needs to change her attitude, but I get where she's coming from.
Once you know more about her past, her current demeanor makes a lot more sense.
I don't agree with it, but I understand it more than most.
Which is why I don't blame her, since I know how she feels, but at the same time I won't let her go too far.
Why make an enemy when you can make a friend?
(even if "friend" is a bit of a stretch at times...)