I thought Haye's attitude has been excellent surrounding this fight - showing a desire to improve, 'I'm not the finished article' that kind of thing. The performance was exciting and effective, against a credible opponent, but as you say, a bit sloppy.
Vitali, may have a better chin than Wlad, but he's the easier fight. He doesn't have the speed or the punching power of Wlad.
I don't see why this should be considered that big of a factor, concerning a fight between Haye and one of the Klitshckos. Haye's basic plan won't be to KO his opponent, but to hit and move. Wlad's defense is excellent right now, and he's by far the most dangerous puncher of the two brothers. Plus, Vitali is on the decline, Wlad is on his peak right now. A fight vs Vitali is more winnable for Haye imo, but I don't think he would risk losing vs Vitali and thus not having a fight with Wlad. Plus, Wlad is the one craving to fight Haye more, so if Haye fights a Klitschko, I'm almost certain it would be Wlad.