But Orlando Pownall QC, representing Cairns, asked Detective Chief Superintendent Michael Duthie whether Vincent was ever at risk of prosecution, or was the warning just a "sham" to bolster his credibility.
Mr Duthie said Vincent was not granted immunity or offered assurances he would not be charged in exchange for giving evidence against Cairns.
He said the investigation focused on the allegations of perjury in the High Court in London - inside his jurisdiction - not match-fixing in cricket games in other parts of England or overseas.
The senior detective said the matter was referred to the Met by the International Cricket Council and the English Cricket Board, which was going to deal with Vincent's match-fixing through a disciplinary tribunal.
The decision to not charge Vincent for match-fixing was operational, said Mr Duthie.
"I have 450 murder investigations, I'm not looking for more work. If the ICC and the ECB want to deal with it, and I agree the sanctions are appropriate, then that's it."
Vincent later received 11 life bans from cricket.
Mr Pownall also read emails between Detective Constable Lucy Wade and Vincent and his wife Susie Markham, who had written to the police officer about concerns he might be arrested if they flew to the UK to visit her family.
The detective said the police had "confirmed at a senior level" that no law enforcement agency intended to investigate Vincent for the crimes he had admitted, but added the caveat that the position could change if new information "came to light".
Mr Duthie confirmed that was the position adopted, but denied that was because the police feared Vincent would not cooperate as a witness if he was charged.
"My decision was to investigate what happened at High Court, not matchfixing," said Mr Duthie referring to the libel trial between Cairns and Lalit Modi.
"I knew [Vincent] cheated and lied. We didn't give any immunity or promises. If he started giving evidence about criminality [other than what he already admitted] in interviews with our staff, we would have stopped and cautioned him."
But the earlier advice from the Crown Prosecution Service, was that ignoring Vincent's role in the match-fixing in the United Kingdom might seriously undermine the case against Cairns.
Notes from a meeting in February 2014 showed the CPS was concerned about whether Vincent was diminishing his role and "rushing to the door to get in first", so the police needed to find out the full extent of his involvement.
The CPS said the investigation should not be limited to the perjury and perverting the course of justice allegations, which could "seriously undermine the prosecution".
A later email contradicted this advice, saying it will come "would come down to who the jury believed".
Cairns is expected to give evidence next Tuesday. Around 30 people left the courtroom disappointed that the former New Zealand international was not in the witness stand today, although one fan did manage his get his autograph outside court.
- Jared Savage in London
