If they're doing the same LI system as in DA2, I now think that Vivienne won't be romanceable, as Gaider said he was writing two romances and one of the characters he's written is Cassandra, which gives a very strong implication that she's one. And if there's only one other, Sera seems much more likely than Vivienne.
You think it'll be similar to Wynne? I don't know, I think she could be romancable. I think her personality is geared for players who prefer the status quo of the Circles, a polar opposite of Anders.
However, I'm thinking choosing her as an ally might involve being on that ideological spectrum of the mage conflict, since I don't think she's mandatory. If she doesn't agree with the rebellion, she might be sought out by a pro-templar Inquisitor who shares her views about the Circles being under Chantry control.
While some have noted her fashion being a possible issue of conflict, I'm thinking that her Orlesian mindset would be a source of friction with Vashoth and Dalish protagonists (given the culture clashes between the two), even moreso if the Inquisitor from one of those two racial options is a mage.