1st to instant 5th heightening.

[Con578]

In the prologue of warbreaker, we see where vasher gets his breath for this book, along with this small paragraph

“Vasher could, of course, have up to the fifth Heightening at any time, if he wished. That would require certain sacrifices he wasn’t willing to make.”

So what is this? Do we know and did I just forget? How would he do this?

[Pagerunner]

That's his Divine Breath, which he would have to stop hiding.

[Con578]

alright,thanks

[Weltall]

Yeah, Vasher has learned a trick to supress his Divine Breath so that he doesn't get the benefits of those Heightenings (the life sense, perfect pitch, color sense etc...) but he still gets kept alive, so long as he has a Breath per week to feed it. Brandon has said that it has to do with a trick of self-perception but we don't know exactly how it works yet. There were some hints of this before the reveal at the end but they're subtle. Later in the book where Vasher and Vivenna are rescuing a girl who was being held captive to pressure her father, as Vivenna looks at him after the fighting she thinks that he looks bigger than before, stronger. The annotations mention that this is because Vasher's control slipped slightly during the fighting and some of his Returned nature started manifesting before he got it back under control. Similarly, the way the girl reacted to Vasher was meant to be a hint, but not an obvious one.