In his summing-up at Southwark Crown Court, Justice Sweeney pointed out the "potential danger" that Vincent, who was last year banned from cricket after admitting accepting money to under-perform, "might have his own interests to serve".
Vincent secretly taped the Skype conversation in 2011 with Cairns' co-defendant, Andrew Fitch-Holland, that forms the key piece of evidence in the charge of perverting the course of justice that both men deny. He has already admitted to corruption, fraud, money laundering and bribery in England. He has also confessed to match-fixing and, as the judge noted, has previously attempted to lie his way out of trouble.
"In his case, you should be particularly cautious that he might have his own interest to serve," the judge told the jury.
"The prosecution say that nevertheless you can be sure he is telling the truth when he tells us that Mr Cairns recruited him to take part in cheating.
"The defence say that Lou Vincent plainly has an interest of his own to serve, namely making allegations about a big name in the game in the hope of getting away with what he has done, or minimising its consequences, and that he has succeeded in doing so, as while he has received life bans from playing cricket, he had already given up cricket, and he has not been investigated or prosecuted for the undoubted crimes he has committed in this country and elsewhere.
"It is incumbent on me to emphasise to you the potential danger posed by such a witness and point out to you that he might have his own interests to serve by giving evidence.
"What you make of his evidence is entirely a matter for you, whether you are sure that he's telling the truth or not."
The charge of perjury, which Cairns denies after stating during his libel action against Lalit Modi in 2012 that he had "never, ever cheated at cricket", requires the evidence of more than one witness to be accepted as true.
Nine witnesses were called in the course of the trial, but the judge said that the evidence of at least two of the three key witnesses would be required to justify a guilty verdict. In addition to Vincent, the other two are Brendon McCullum, the New Zealand captain, and Eleanor Riley, Vincent's ex-wife.
If the jury was sure that Cairns had cheated, then they would be open to convict on count one, perjury, and go on to consider count two, perverting the course of justice. However, if they acquitted on the first count, then they would be obliged to acquit on the second count as well.
The judge will continue his summing up on Monday at 10am.