Let me start firstly by saying two things.
1. Vettori is without doubt one of the top five ODI bowlers of this generation and a guy who has the right mixture of speed, flight and variation to tie down batsmen.
2. Vettori is without doubt one of the top six batsmen currently available in the longer form of the game for NZ Cricket.
His inability to turn the ball consistently in test cricket, combined with his mentality which seems to be tying down batsmen rather than attacking them is what kills the NZ side in the test environment.
Vettori bowls the most overs, takes the most wickets and goes for the lowest run rate of any NZ bowler, all good qualities - until you realise that he's taking each wicket at about 40-45 and that his strike rate is up near 100 per wicket.
This means for all that time Vettori is restricting someone to 2.5 to 3 an over, he's not really regularly taking wickets or threatening the batsmen, meaning pressure is on at the other end to do so.
My view is that because of this, NZ has regressed as a test playing nation because at least before the day of Vettori (or more importantly, with Vettori before his back problems that curtailed the spin he used to generate), we had more bowlers averaging sub 30 with the ball (Nash, Cairns, Doull, Tuffey in his pomp) than we do now, potentially because they had to bowl more overs because they didn't have Dan the workhorse clogging up an end with efficient yet unattacking bowling.
It's a harsh comment on Vettori, some might say he's shouldering the full burden of the fact that NZ don't have good bowlers at the moment but my view is that it's because he bowls so many thrifty overs that the pressure is put on at the other end and that's where they score runs.