Based on statistics, Vettori definitely comes out tops at the moment. He's taken the most wickets - 13 - and has by far the best economy rate - 3.21. In fact the only bowler with a better economy rate in the tournament thus far is Michael Clarke, and he's only bowled 5 overs.
Well Vitorrei is the most impressive base on statistics. I don't think this world cup has been dominated by spinners because of the pitches. It is the fast bowlers who have done well this tournament.