Vasher Explains Some Things, but Leaves Some Things Hidden
I'm worried about leaving Vivenna's two questions unanswered. One is pretty obvious—how Vasher can hide how he looks—but the other is unintuitive. I wish I could explain better in the book, as I said above, but I decided in the end to just leave it hanging. It's a bit of a violation of Sanderson's First Law, but not a big one. The reason I feel I can get away with it is because Vasher didn't use his nature as a Returned to solve any problems. It is more a flavoring for his character than it is important to him getting out of danger or fixing things. He could have done everything he needed to in this book without being Returned. So I feel it's okay not to explain why he can be Returned and not die when he gives away his Breaths.
Can Vivenna change her appearance more? She can indeed. She could actually stoke that fragment of a divine Breath inside of her and start glowing like a Returned. She can't change her physical features to look like someone else, but she can change her age, her height (within reason), and her body shape (to an extent). It takes practice.
And yes, the scraggly miscreant is how Vasher sees himself. Not noble and Returned, which is part of how he suppresses his divine Breath.
Events in the second book may change that.