Easy, Kallis. As good as Vettori is, Kallis has been a world class player for years. He's a middle order batsmen who has 20 odd 100's and averages in the mid 50's, factor in his bowling he's one of the best players of his time.
As for Vettori batting higher in the order, it's just not possible with this New Zealand team, the top three is just far too weak. If we had McCullum six and Vettori seven we'd pretty much go from having a great tail to having one that will barely score 50 runs between them. Vettori's fine where he is, since Janurary 2005 he's averaged mid 40's with the bat and really gives this New Zealand team an edge with our usually strong 6-7-8, and sometimes nine.