"I feel relatively confident about that but it's always a moving topic.
"I think at my age and with how much I've played, I'm always a little bit sceptical about giving it an all clear."
Vettori has been working with conditioning coach and former New Zealand sprinter Chris Donaldson which he said has helped a lot.
Hesson said they'll also manage Vettori's workload after his inclusion in the squad yesterday.
Batsmen Dean Brownlie (three ODIs) and Tom Latham (13 ODIs) have been handed the chance to stake their claim for an opening spot following injuries to Ross Taylor and Kane Williamson. Taylor is expected to miss the entire series with a calf strain while Williamson will miss the first match with a wrist injury.
The bowling attack will see Trent Boult - whose last one-dayer came in February 2013 - attempt to prove himself with the white ball, while Tim Southee has been passed fit after being unavailable for New Zealand XI's rained-out match against Ireland on Saturday scheduled for Seddon Park in Hamilton.
Hesson said Southee is continuing to recover from a shoulder injury and they may rest him for game one tomorrow.
"If he's not at full capacity then he won't play the first one, but he'll certainly be available to play two and three and should be firing for it by then," Hesson said.
"He's strong and fit and he recovers quickly. So this time of year it's not an opportunity to push guys too hard too soon."
Squads for ODI tomorrow
New Zealand: Brendon McCullum (c), Corey Anderson, Trent Boult, Dean Brownlie, Martin Guptill, Matt Henry, Tom Latham, Mitchell McClenaghan, Nathan McCullum, Kyle Mills, James Neesham, Luke Ronchi, Tim Southee, Daniel Vettori.
South Africa: AB de Villiers (c), Kyle Abbott, Hashim Amla, Quinton de Kock, Jean-Paul Duminy, Faf du Plessis, Ryan McLaren, David Miller, Morne Morkel, Wayne Parnell, Aaron Phangiso, Vernon Philander, Rilee Rossouw, Dale Steyn, Imran Tahir.
Key numbers
169: The highest individual score in an ODI between South Africa and New Zealand, scored by Dave Callaghan at Centurion in 1994.
33: Chris Cairns owns the record for most ODI wickets taken by a New Zealander against South Africa. Kyle Mills is two behind on 31.
20: ODI victories for New Zealand over South Africa from 58 matches.
9: The number of those wins that were on New Zealand soil.
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