Lightsong an Idrian..... hmmm possibly. So then his visions/dreams/flashes of memory are foretelling what is to come?
Well, Vasher could find/steal/buy breaths just about anywhere in Hallendren to kidnap Vivenna for the soul purpose of stealing her breath is a bit thin. Especially, since he can acquire it with ease had he wanted too. But he is not the type to do things that are easy. In chapter one he got himself arrested to get the breath of the man, Pahn, and kill him. Seems there was more to that than we currently know. But again my point is he raised the difficulty level for the want of breath alone... I don't think so. There is more to him than that.
Denth hates him and not for just killing his friend and perhaps Denth/Vivenna's plans are working against Vasher's he takes her out of the picture all that much easier for him. Not that he was successful, but that's just foreshadowing for you. There is so much more that can happen for good or ill.
Though, Vasher gives off mixed vibes on the good or bad thing. Doing the right thing for all the wrong reasons, perhaps? Does he have to be good or bad? I mean at first I was all ready to hate Susebron and you cannot help but feel compassion for the man... oops God King. Not something I had anticipated - loved that twist.