With this, do you think she's out of character to allow the College and Circle to co-exist?
No, but more for the fact that it would be in her character to do so depending on circumstance than for how it was briefly described in the epilogue slide. The slide portrays the choice as an act of magnanimity and a favor to the Inquisitor... even if the Inquisitor is anti-mage freedom.
What the slide doesn't say, but other context does, is that Vivienne has enough problems consolidating her rule and keeping things together, that preventing the Circle might not be a fight she's willing or able to take on at the moment. By trespasser she's just coming off of multiple rebellions, her her own mage reforms to deal with, and is facing a Qunari crisis. Mage dissent might not be the easiest thing to suppress, especially considering it's not quite clear if all the mages who fled during the mage rebellions have actually returned, or are still apostates in hiding.
There is a logic to allowing the College to exist, and it's much the same as what many dominant political actors or parties do- permit a marginal opposition group to exist, even especially when it's not a real challenger. There's multiple benefits to doing so: it can be used to demonstrate that claims of totalitarianism are overblown (e.g., Vivienne being a tyrant even as she allows her most bitter mage opponents autonomy), it offers a safety-valve for anti-establishment forces to try and agitate through in a more predictable manner, and it invites dissidents and political opponents to self-identify and gather all together in a way they can be watched and observed. There's a reason why the staunchly anti-communist United States of America allowed the American Communist Party to survive during the Cold War.
Of course, there's another angle as well- the popular discreditation and deligitimization of the College proposal. Quite rarely is political defeat alone enough to end a movement- otherwise, inevitable setbacks would doom all great ideas. Instead, if you really want to stop something you have to convince enough people that it's such a bad idea that it shouldn't be done in the future.
If Vivienne outlaws the College, mages can whisper and conspire to bring it back, convinced they would succeed if given a chance. If the College fails on its own accord, however, or in competition with the Circles, then that insurgent sentiment will be undercut. It's not enough for the College to be banned- it needs to lose, popularly, objectively, politically, to the point that mages wouldn't want to join the Circle. If they want privileges, security, advancement, and protection, they'll go to the faction that offers the better mix. Ultimately, the Colleges will fail when mages give up on them.
Now, that's not necessarily a bad thing. An objective pro-mage, as opposed to an anti-Circle advocate, would probably rejoice in that sort of competition- either it means Vivienne is being forced to do more than would otherwise be done, or (equally viable) it means it gives Vivienne cover to do what she already wants to. Vivienne can justify her reforms to even anti-mages on the grounds that if she doesn't, the more dangerous Colleges will grow freer.
Of course, there are other things as well, not necessarily good factors. Competition can undercut eachother, and nothing will ensure the Colleges fail better than, well, ensuring the Colleges fail. Zero-sum competition. And, of course, some fundamental disagreements about what sort of freedoms and oversights mages should have.
But that's natural. And, I suspect, inevitable. No matter what Divine you have, you'll have a College and you'll have Vivienne supporting the Circles in contest for public, and mage, support as she works to not only render the Colleges irrelevant, but to rob them of even mage support.
And, to reach back to your original queation, I think that that would be quite in character.