Is it really that bad?
I enjoy her snark for the most part but the one time I actually wanted to slap the taste out of Viv's mouth was when she chimed in while Dorian was giving Solas grief over his choice in clothing with her "unwashed apostate hobo" remark. 
And yeah, I get that comment is pretty tame compared to some of the other stuff she spouts off but that ruffled my feathers. I can't help it. I
Solas.
Blackwall: Are you alright, Lady Vivienne?
Vivienne: Whatever are you talking about, darling?
Blackwall: In our last fight, I saw you take a blow. If I'd been there quicker—
Vivienne: Oh, aren't you precious?
Blackwall: I appear to have offended.
Vivienne: No, dear. You couldn't possibly offend me.
Blackwall: What fault do you find with me, Vivienne? I've tried to be civil.
Vivienne: Darling, your civility has so little to do with your faults. I wouldn't let that trouble you.
Blackwall: Then what am I doing wrong?
Vivienne: Just keep hitting things, my dear. Don't worry your little head about the rest.
Blackwall: Evidently, I offend you just by existing, Vivienne. What is it you want me to do?
Vivienne: I'm sure I don't know, darling. Do you imagine I spend all my waking hours worrying about your self-improvement?
Blackwall: Look, I wasn't trying to imply that you--
Vivienne: My dear Blackwall, there's nothing you can do to make your Order relevant to our present situation.
Vivienne: You really must stop assuming everything is about you. It's unbecoming.
Yeah, she can be pretty mean at times to Blackwall and it's never really understood why because she still acts this way even before his reveal.
Perhaps Fiona made her just hate Grey Wardens?