It was said throughout the book that you cannot just give some of your breath, but must give all of it. Perhaps I’m simply forgetting part of the book where this changes, but wouldn’t Vasher have to give his Returned breath along with his others?
The “You must give up all of your Breath, not some” line was mostly perpetuated by Denth, who is saying it to Vivenna to stop her from giving away her Breath to all the people she passes. It is a lie. Now, it’s a lie that’s commonly accepted by a lot of people. But it’s still a lie—as we find out midway through the book, you can stick some of your Breath in an object and bring it to life, and then recover that Breath. So it’s very easy to give some of your Breath to someone if you know the logical steps to take. Invest most of it into an object, give what you have to someone else, then pull back what you Invested. So it’s flat-out proven in the novel that what Denth is telling her is wrong. Now, he could dance around that lie by pretending to be the ignorant mercenary—he’s just perpetuating a falsehood that many people believe. But it is a lie. In fact, a lot of the things people believe about BioChromatic Breath isn’t true.
One of the things I was trying with this book was to take a few steps back from MISTBORN, where so much was understood. I feel that the approach I took in MISTBORN is right for that book, and yet people have so much superstition regarding all sorts of science. I worry sometimes that there isn’t enough superstition in my books, regarding magic as science. What people believed and what people knew and what people understood was so varied and confused throughout most of history, that I worry that I lack realism in that. Vasher brings up at several points in the book that they don’t know a whole lot and that people perpetuate a lot of myths and stories and lies.
Vasher has learned to suppress his Returned Breath. When it’s suppressed, it’s as if it doesn’t exist to him. He’s Invested it into a place within himself, much like you can Invest your Breaths into a shirt, and when he gives away the rest of his Breaths, he doesn’t give that one away. He could split off others of his Breaths if he wanted to—he’s learned to do that, so that he could give a few Breaths and not all. It’s just a matter of practicing as long as he has. But even people who aren’t as practiced as him do it all the time when they Invest an object with not all of their Breath but just enough to bring it to life.