Darryl Proctor was convicted of inflicting GBH with intent
The father of a Londonderry man attacked in July 2006 has said there is "a potential murder gang in the city".
Paul McCauley, 33, was attacked at a barbecue in the Waterside and has never regained consciousness.
On Wednesday, Darryl Proctor, 19, who had previously admitted inflicting grievous bodily harm with intent, failed to have his sentence reduced.
Proctor, from the Fountain in Derry, was 15-years-old when he and up to five others attacked Mr McCauley.
Mr McCauley's father James said the others involved in the attack had to be pursued.
"When you see your son not being able to move, not being able to communicate, being fed by a tube into his stomach, in need of permanent nursing care, the people that carried out an attack like that should not be allowed to walk free," he said.
"Paul was left for dead and had to be resuscitated on three occasions."
Proctor's lawyers wanted his 12-year term cut by a third so he could serve it all in a young offenders' centre.
They had argued that the sentence was excessive and said more weight should have been given to his relative youth, the guilty plea, and the fact that others were involved.
However, the Court of Appeal in Belfast rejected the case and ruled that the original sentence had been neither manifestly excessive, nor wrong in principle.
Lord Justice Coghlin said Mr McCauley was still in a minimally responsive state.
"He has to be fed through a tube and is totally dependent upon nursing staff and carers," he said.
"There is no potential for any recovery at this stage. He will remain in a low-level conscious, probably vegetative, state and will require full-time care for the rest of his life."
He added that Proctor had taken part in a "totally unprovoked sectarian assault" that resulted in appalling injuries sustained by Mr McCauley.
"While we accept the sentence was severe, the courts have a duty to respond to such sectarian violence by imposing sentences that are severe enough to sufficiently mark the total abhorrence of law abiding society and adequately comply with the requirements of deterrence and retribution," he said.