To be honest, this isn't really clear. The customization in DA:I showed in a demo is what I expect to see the most. Because it's not clear how a gear from Blackwall for example could go on Vivienne.
Well, I'm still unsure if companions will have access to armours outside their class.
Well this concept art gives us an idea of what she looks like under the hat. Unless they changed it after.

I wonder if any of these outfits will appear in game. The default one that they picked is actually my favorite but it would be nice if we got to see her in some of the others.
We know that at least this one was actually available at some point. 
I also want to know what actually happens when we put other outfits on her. They are meant to change to fit her style but how much will they change?
If it does still work like that and armour skins of these concept arts all exist in game, I'd imagine that the four variations on the end will be for light/mage mage armour, the two feathery ones will be the medium/rogue armour and the heavy/warrior armour will be represented by the armour we've seen Vivienne wear most commonly and the one beneath it.