Zack's undying optimism is something I really love about him - and the fact that it's infectious is wonderful thing.
And yes, I think everyone in the afterlife will know it the moment Vash shows up (am sort of picturing the equivalent of air raid sirens and disco lights), although whether Wolfwood will ever follow Zack's example is hard to say. (The thought of him and Zack trolling around hell is absolutely awesome though).