She's a victim of the 'Wicked Eyes and Wicked Hearts' syndrome, where Bioware set characters and situations up to be full of deadly, political intrigue and machiavellian masterminds etc etc, but their writers aren't nearly comfortable actually executing that. Her character should be divinely elegant, with elements of cruelty and subtlety, with inscrutable motives who will constantly surprise you with either sheer ruthlessness or unexpected compassion etc etc.
But instead she just comes across as a tiresomely snobbish, overtly superior and haughty character, who basically spends 99% of the game, insulting people to their faces because she feels entitled to do so and/or knows they can't or won't do anything about it. She is less nasty to your character (If you agree with her anyway), but only because she is using you in a very obvious way. I just saw her as an even more irritating version of Morrigan. In the case of Morrigan, that character can sometimes reach (to quote a description of Ace Attorney character Franziska Von Karma), a state of being 'So openly hostile, its actually quite endearing'. And her cat and dog fighting with Alistair is great (because whilst Alistair usually loses, he does occasionally get a win over her).
But Vivienne... eh. And I was playing as a character who basically agreed with everything she said about the Circle, if not her attitude. I just kept thinking of what Sherrif Harry Truman of Twin Peaks says in an early episode to Dale's outrageously abrasive and rude FBI friend - 'People who come round here, talking this kind of crap, will pretty soon be looking for their teeth, somewhere up on Queer Street'.
I.e, its hard to see why people don't just say 'Oh, just f**k off!' to her. They just don't sell her a woman of power and influence (magically or politically etc etc). And they continually get this wrong with the nobility in this game. On the war room operations, you hear of and receive letters from nobles who have done good work, who use their station to help those in need, or at least make sure the wheels are turning so resources get where they need to go etc etc etc. But all the nobles you *actually* see in game, are just preening fools. Take 'Champions of the Just', where Orlais thinks sending obviously spoiled and out of touch nobles in ballroom finery to meet with the down to earth Templars, who are waging a war on matters of principle and uuty.
Its at odds with what Aveline points out in Mark of the Assassin, where she urges the party not to judge Orlais too harshly because of all the foolish nutcases they see at the hunt. Because she says 'Not all Orlesians are this silly - my father wasn't. and if I'd stayed, I wouldn't have been either'. Its just a symptom of how steretypically Bioware write such characters. Spies also come across very poorly, with all spies and assassins being a variant of a Bond Girl (and yes, that includes Zevran). They simply don't have the will or the interest to provide a selection of different personalties beyond simple 'pompous and ignorant', 'Nasty and self important', 'silly and ditzy' etc etc etc,
Vivienne falls into this trap. She is a very stereotypical noble who's always chuntering away about how uncouth and ill bred everyone is, how the others should know their places, how she's doing them a huge favour even letting them think their opinions on anything matter etc. And yet we never see any examples of why she should be accorded any respect, not that the way she speaks to people would be acceptable regardless fo what she was capable of.
As one of my rather blunt mates put it when he heard her speak 'Shut your gob, you snooty cow!' LIke I say, a bit blunt but I can't say I disagree with that sentiment in this case.