Sentencing judge Stephen Walmsley said the pair were motivated by a belief the man, who had connections with rival bikie gang the Rebels, had been disrespectful to Vimahi's uncle.
He called the March 2015 attack a "wrongful attempt at vigilantism".
Prosecutor James Walker said: "Their botched shooting was a violent escalation of rivalry between the two gangs, and their vendetta against the victim."
The judge said both men had since showed a lack of empathy for the victim, who had to have surgery to remove a bullet from his body and spent several days in hospital after the shooting.
Grech and Vimahi had been charged with intentionally inflicting grievous bodily harm, but their barristers at trial had convinced the judge to find the pair guilty of the less serious wounding, which carries a maximum penalty of five years jail as opposed to 20.
This was because in the months after the attack their victim, who was in and out of the Rebels, had launched a vigorous and sustained attack on his ex-partner, in which he had repeatedly raped and assaulted her and thrown her around her home.
The judge said while the man the bikies had shot suffered serious wounds, he agreed it did not amount to grievous bodily harm.
The court heard Grech had been disciplined for poor behaviour while in prison, and Vimahi had shown ongoing animosity to others.
Grech's younger brother gave evidence on Wednesday, telling the court he noticed changes in Grech after their grandmother died, and he also suspected his brother may have been on drugs.
But he had also been visiting Grech at the Alexander Maconochie Centre and felt his big brother had become himself again.
"The Daniel that I know now is the Daniel I knew growing up."
Grech was talking about starting from scratch, saving money and building a career once he leaves jail, his younger brother said.
The judge also sentenced the two men for charges relating to the possession and storage of firearms and ammunition, and Grech over a stolen car, to which both had earlier pleaded guilty.
Both men have been in prison in Canberra since 2015.
The judge gave Grech a term of six years and six months in prison, with a non-parole period of three years and three months. It means he'll be eligible for release in June 2018.
Vimahi was handed a sentence of six years and two months, with a non-parole period of three years and two months.
He will be eligible for release in October 2018.