That's the thing. It might be against Chantry doctrine, but not the Chant of Light. There's still a difference. And listen to Cassandra in the beginning at Roderick. She doesn't follow the Chantry either. But the principles on which the Chantry is founded: that is, the Chant of Light.
Chant of Light: Magic exists to serve man, and never to rule over him.
Vivienne's rule:
"Given the name Divine Victoria, she is the first mage to hold the position - and the uproar is immediate."
"Three revolts follow in quick succession, and her response is immediate and vicious. For months, chaos reigns."
"To the surprise of many, she reinstates the Circle of Magi and creates a Templar Order firmly leashed to her hand."
"Mages rise quickly in the new Circle, having more freedom and responsibility then ever before - even if all true power lies with her."
It refutes the Chant of Light in the same manner that the Tevinter Divine refutes the Chant of Light.
Other than that, the original Inquisition set this current system in place. Not the Chantry. It's free to change it.
Oh? Isn't that the same hijacking of a religion with millions of followers that you were railing against on the last page? To what extent, exactly, are we allowed to change it before it becomes "humiliated and broken"?
As for the other stuff, I still disagree that it was power play to stick with the loyalists. I think she believes what she says. That magic is dangerous and must serve better purpose. And she still maintains that as Divine. There's no sign of her becoming like Tevinter.
Vivienne believes in what is best for Vivienne. Her personal beliefs regarding magic just happen to be convenient to this end.
While of course she won't try to turn southern Thedas into Tevinter (after all, why would she?), her position as Divine would make any Magister green with envy.