"Sri Lanka played exceptionally well last night to beat them. We are under no illusions about how hard it is going to be."
Sri Lanka defeated the hosts by 55 runs in a rain-hit match at Centurion on Tuesday to boost their chances of qualifying for the semi-finals. New Zealand and England are the other sides in the group.
The top two teams advance to the semi-finals.
"We know coming here that we have to beat quality teams," said Vettori, whose side clash with South Africa in their opening match at Centurion on Thursday.
"England have come off a tough series against Australia. But all teams in the group will be wary of each other."
New Zealand go into the match in a confident frame of mind, having beaten India in a recent warm-up game.
"We had mixed results in the warm-up games. We lost to a first-class team and played really well against India. Hopefully, we can take the confidence of the India game into our first match, particularly our batsmen," said Vettori.
"To make 300 against an Indian side is always something that makes me happy as captain and pleases the team as well. The challenge is to put a score on the board and defend it as a bowling unit."