That actually goes to why I think Vivienne and Sera are so unpopular; they're very static characters. They never really develop beyond how we meet them. We get a few peaks behind the curtain with them; Sera's cookies and Vivienne's Bastien, but once those scenes are over, the masks are back up and it's like that's never happened.
Most all the other characters have defined arcs. Even other unpopular or divisive characters like Velanna and Morrigan undergo clear character development. Vivienne and Sera really don't.
Here, here! Couldn't have put it better myself!
This was my biggest problem with Vivienne's character in this game. I desperately wanted to like her and become invested in her character, but I never could because she simply had no arc whatsoever.
Sera at least had some funny moments to make you like her (and I do) and had something of an arc in that being around the Inquisitor and all their mad hullaballo was starting to make her question her faith and what it meant to her? Plus her romance arc allowed you to see a side of her that actually took something seriously for once and showed how much her relationship with the Inquisitor meant to her.
Vivienne's quest would have been so much more effective if we'd maybe one or two more quests to find certain things for her and gotten to see a more human side to her during those missions, only realising at the end that all of this was part of the potion required to save Bastien's life. As it was, I sympathised with her during that scene, but I didn't really have a reason to care about her afterwards because she goes right back to being an Ice Queen.
It's a shame because I adore Vivienne as a character and her VA is awesome, but she simply never did anything interesting to make me emotionally invested in her and her goals. As Blackwall so aptly put it, it seemed like she joined simply because she wanted to use the Inquisition to grab power.