Vincent, a former Black Cap and confessed match fixer, was the first witness to give evidence at Cairns's perjury trial at Southwark Crown Court in London.
He told the court he became involved in match fixing in the 2008, and cheated on direct orders from Cairns in a number of cricket games.
The Crown says Vincent came to the trial a "broken man".
"He was humble, he was contrite, and he was honest to you. He bared his sole," prosecutor Sasha Wass QC said in her closing remarks last week.
"He has not tried to help himself by blaming Chris Cairns. He has not profited by blaming Chris Cairns."
However, the defence says Vincent has lied repetitively about his involvement in fixing and has continued to do so in the current trial.
It says Vincent handed over a big name to authorities to minimise his own dishonest behaviour and is planning to write a tell-all book about match fixing with Cairns.
In his direction to the jury on Friday, Justice Nigel Sweeney warned that witnesses might have reasons of their own for giving evidence. And he singled out Vincent.
"In Lou Vincent's case, you should be particularly aware he might have his own interests to serve," the judge said.
Justice Sweeney noted that Vincent had committed offences for which he had not been prosecuted, and such a witness posed a "potential danger".
Before giving evidence, Vincent was cautioned that anything he said in court could be used against him in a later prosecution.
However, in his comments on Friday, the judge said it had become clear in the course of the trial that such a caution was unnecessary and it should not be seen as lending credibility to Vincent's evidence.
"In reality, Lou Vincent was not at risk of public prosecution in this county for any crimes he had admitted up to the point he gave evidence," the judge said.
"In any event, had the court known that at the outset of Lou Vincent's evidence, the caution would have been different."
The judge will continue his summary on Monday, and has told the jury it is likely to retire to consider its verdict on Tuesday.
Cairns is charged with perverting the course of justice and perjury in relation to a 2012 libel trial in which he stated that he "never, ever cheated at cricket".
