He was batting with former Australian international Damien Martyn for the World XI team, chasing the total of 148 set by the India XI.
Vincent told the jury the World XI was 62/1 and he was on "16 or 17" [records show he was on 17] when he decided to get out.
He said it was a "disaster".
"I can tell you exactly what happened, it feels like it happened yesterday," he said when Crown prosecutor Sasha Wass QC asked him what happened.
He was about to face left-arm spinner Ali Murtaza when he decided to lose his wicket by getting stumped.
Vincent charged down the pitch, "but the ball did something funny" and he smashed the ball back over the bowler's head for six.
"My heart just raced, how did this happen?" Vincent told the jury.
He tried to miss the next ball in the hope of getting bowled, he said. Instead he edged the ball which raced to the boundary for four.
"I was supposed to get out, I've just hit 10 runs, this is a disaster," said Vincent, who ended up scoring 28.
"It was a fix that went wrong, from my point of view."
Vincent alleged he was supposed to be fixing for Cairns, although the jury obviously didn't believe him.
Cairns, the captain of the World XI, was the next batsman in and wasn't happy as they crossed paths, Vincent told the jury.
"I could tell by the look on his face that he was fuming," Vincent told the court.
"I was anxious, I was worried about letting Chris down. I was under the instructions of Chris and I didn't deliver. And I didn't know what the consequences would be."
In questioning Vincent, Cairns' lawyer Orlando Pownall, QC, suggested the six was a "beaut" and the four was "purposeful", a classic late cut off the middle of the bat.
"You bring the bat back and you deliberately smite it," Pownall said of the six as footage was shown to the court.
Vincent said his "footwork was horrendous", which Pownall disputed.
"I disagree with your cricket opinion," Vincent replied, which got laughs from the public gallery.
In his evidence, Cairns disagreed with Vincent's assessment of the six as a "fluke".
"I think he came down and gave himself room, did nothing but follow through with the shot ... it was a good shot timed very, very well."
Cairns said the four, described by Vincent as an edge, was a late cut.
"A tough shot which came off the face of the bat. That came off the face of the bat, no edge."
Pownall asked Cairns whether Vincent looked like he was trying to get out; Cairns replied no but the shot was aggressive.
"[It] shows me he was taking a risk by exposing the stumps. Very good shot."
Cairns agreed that he was livid as he went out to bat - not because Vincent had ruined the fix, but because he jeopardised his team's chance of victory.
The World XI were 75/1 before Vincent got out. Cairns thought they were in a good position to win and Vincent "should have seen it through".
"I finished up man of the match, won us the game."
When Pownall said Cairns was not involved in match-fixing at all, Vincent retorted: "Maybe you should show the footage of Chris Cairns of batting in that innings."
The footage was shown the next day. The scorecard shows Cairns scored nine runs off 16 balls, and was then run out.
Asked by Pownall if he was trying to score runs, Cairns said yes but it wasn't always possible to be successful.
"A far better judge than me, Ricky Ponting, concurred [in giving evidence earlier in the trial]. It doesn't always go to plan."
READ MORE
• How Chris Cairns was acquitted
• The Skype call that kicked off Chris Cairns charges
• Brendon McCullum's three statements
• Dylan Cleaver: Inside the match-fixing saga