Exactly, I see Nightblood as one of the only ways to truly get rid of the souls of these Ancient Singers who are inhabiting the main power in Odium's forces. Obviously, Szeth and Nightblood can't kill all of them (Right?...) so that's not the whole story here. Remember that the honorspren in Shadesmar mentioned having Nightblood pass through their fortress a while back, as well as the Nightwatcher having told Dalinar that she could give him a sword that bleeds darkness for his boon. I believe that this has been known as one of the strongest weapon against Odium for quite some time, and Vasher probably specifically was contacted to bring it in a desperate time of need.