Depends how you go about it. If you are adding a bowler for a batter then obviously it weakens the batting. But if you use the current NZ lineup, if McCullum is staying at 6 then Vettori can move to 5, Fulton 7 and Elliott 8 which on paper doesn't weaken the batting.
Yeah I guess you could move Vettori up if you play batsmen under him. But how many batsmen are going to succeed batting at seven and eight. Vettori's got a game that works well batting as low as he does.