Vivienne would have a proper education and on top of that is still a "warrior." While Morrigan is a good mage Vivienne has actual combat skills, Morrigan however doesn't. She would probably be able to defend herself, but the moment Vivienne would pull out that sword it would be game over. Morrigan has no defense as opposed to Vivienne who gets a barrier for every hit she makes. She gets bonuses to her constitution and can basically manipulate your speed. Knight Enchanters are still considered warriors don't forget that. People think just because you're Flemeth's daughter it means you inherent all of her abilities, which is not the case. Maybe Viv would lose against Flemeth, but Morrigan is still far younger and far less experienced than Vivienne. Lets be honest what's the only reason people pick Morrigan? Because everyone likes her that's why. Vivienne is unliked because she isn't a rebel mage and prefers to be classy as well as arrogant (yet we like Morrigan?) Morrigan is the most popular female in the game so of course we're gonna vote for her.
...and if not only a one-time thing? No matter. Vivienne only a Circle Mage. A good one, but so narrow-minded, and fears from her own magic too. Morrigan more flexible. She studied the dalish magic and the forbidden magic.