A vet who denies supplying cobalt to trainers in two states says his former boss is using him as a scapegoat.

Forced to give evidence to Victorian trainers Danny O'Brien and Mark Kavanagh's appeals against their cobalt disqualifications, Dr Adam Matthews again denied being involved.

Vet Dr Tom Brennan claims Matthews supplied bottles containing a substance called vitamin complex that Brennan gave to horses in the O'Brien and Kavanagh stables and sent to Kavanagh's son, Sydney trainer Sam Kavanagh.

"Again Tom's using me as a scapegoat," Matthews said.

Matthews left Flemington Equine Clinic, where Brennan was a partner, in March 2015.

He agreed with O'Brien and Kavanagh's barrister Damian Sheales that it was a bitter and acrimonious split, denying dishonesty accusations Brennan made against him.

Sheales said Sam Kavanagh had testified Brennan waged war on him after the trainer revealed the vitamin complex came from Flemington Equine, destroying his reputation, friendships and relationships and nearly ruining his life.

"Are you of the view that Tom Brennan waged the same war on you, with the same outcome," Sheales asked Matthews, who agreed.

"And you would say like Sam Kavanagh you're simply telling the truth," Sheales said. Matthews again agreed.

Matthews said Brennan first used him as a scapegoat when the Flemington Equine partner told stewards in July 2015, six months after Kavanagh and O'Brien were informed of their cobalt positives, that Matthews supplied the vitamin complex.

Brennan has told VCAT he was "hoodwinked" by Matthews and denied making up a story about Matthews sourcing the vitamin complex from Canada.

After failing to have a summons to appear as a witness set aside on medical grounds, Matthews on Wednesday repeatedly told the tribunal Brennan's evidence was incorrect.

Matthews admitted he had a severe gambling problem, which the court heard had involved $4 million in gambling turnover over three years.

Sam Kavanagh and an unnamed harness trainer occasionally paid money for veterinary products directly into Matthews' wife's TAB betting account, VCAT heard.

Sheales repeatedly interrupted Racing Victoria barrister Jeff Gleeson QC's questioning of Matthews, saying the vet had no evidence that assisted the stewards.

"It looks to me like just a smash character assassination," Sheales said.

"It's a disgrace. They've pulled him out of a mental health institution to come and give evidence that he's given three times before that they know."

Matthews' barrister Robert Richter QC suggested Racing Victoria was trying to help NSW stewards in their upcoming appeal of Matthews' acquittal in the Sam Kavanagh cobalt case.

"This whole cross-examination looks to me like it is designed to assist the NSW stewards in their appeal against his acquittal," Richter said.

Matthews was originally disqualified for 5-1/2 years over the Midsummer Sun cobalt case, but the Racing NSW Appeal Panel found Matthews not guilty of being the supplier of cobalt in the vitamin complex bottle.