^Gonna have to disagree with you there. To me, Vivienne a lot more compelling than Morrigan.
I romanced Morrigan, and can identify with her character a lot. Still, her character has never been stellar, in my eyes. In DA:O she is new to the outside world. Her perspective is unique, but kind of irrelevant, since she knows basically nothing about how things work. She is not known to be interested in anything beyond protecting herself from Flemeth, which is not very interesting. Quite frankly, I believe she is overrated, with her exotic sex-appeal and Claudia Black voice-acting carrying her. I cannot find too many other reasons to be high on her.
With the OGB arc I thought Morrigan may have been destined to some great things and would be revealed to have some interest in the world beyond her own survival. Come DA:I, however, that plot thread went nowhere meaningful, and it looks like Morrigan's story is just about over.
Vivienne's character begins, to our eyes, where Morrigan's basically ends: a ranking mage in the Orlesian Court. It potentially ends with her being a truly revolutionary figure in Thedas (a mage Divine), which was what my hopes for Morrigan were before then. As such, Vivienne has become one of my new favorite characters, whereas Morrigan is somewhere in the middle of the road for me (like her, don't love her).