We really have very little info about this sort of thing and I think people are thinking about Vivienne a little too hard. Personally, I think her entire "agenda" is just to use the Inquisition to get further in the social hierarchy. Her being an LI or not has no bearing on this, as Morrigan WAS an LI and she had an ulterior motive and pretty much betrayed you.
I agree. Vivienne is ruthless and ambitious, two things you need to have to survive in the Orlesian court, along with intelligence, attentiveness, and an impeccable talent for turning a phrase. I don't really like those two things about her, but then again, I'm not a fan of the Orlesian court, either. She's someone to watch, to be sure, but I don't think she'll betray us.
Rather, I think her "hidden agenda" more geared towards solidifying an expansion of how much political sway mages are allowed to have in the Orlesian courts. She might want the Circle to return, but that doesn't mean she doesn't want to expand its influence, and as former First Enchanter of the Montsimmard Circle, that would count for a lot.
I'm actually looking forward to getting to know her. She might actually surprise me, and be an invaluable member of the team. She'll definitely act as a foil to my very non-ambitious, fashion-ignorant Qunari mage...
Vivienne: Darling, are you absolutely sure you want to wear that? 
Inquisitor: What? It's comfortable and protects most of my vitals. Why wouldn't I want to wear it? 
Vivienne disapproves (-10)
Inquisitor: Vivienne? What's with that glare?
Vivienne disapproves (-15)
Inquisitor: Oh, come on! What did I do?! 