Daryl Proctor, 19, admitted inflicting grievous bodily harm with intent on Paul McCauley at a barbecue in the Waterside in July 2006.
Mr McCauley, 33, has never regained consciousness.
Proctor's lawyers wanted his 12-year term cut by a third so he could serve it all in a young offenders' centre.
Proctor, from the Fountain in Londonderry, was 15-years-old when he and up to five others attacked Mr McCauley.
His defence 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 today rejected the case and ruled that the original sentence had been neither manifestly excessive, nor wrong in principle.
Lord Justice Coghlin said that Paul McAuley had been subjected to a concerted assault involving kicking and stamping causing a fracture of his skull.
The judge said Mr McAuley was still in a minimally responsive state, unable to vocalise or verbalise.
"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."
Lord Justice Coghlin said the attackers had left a primarily Protestant area to go to a predominantly Catholic one.
"No one is born with sectarian attitudes or beliefs," said the judge.
"Rather they are transmitted as a consequence of exposure to warped and malign influences exercised by the culture and environment in which individuals are raised.
"Violent attacks such as this are likely to be repeated until such influences have been completely eradicated by the communities in which they have been hitherto allowed, if not encouraged, to flourish."
Lord Justice Coghlin said Proctor had taken part in a totally unprovoked sectarian assault that resulted in appalling injuries sustained by Mr McAuley.
He added: "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.
Outside the court Mr McAuley's father, Jim, said: "We are satisfied that the sentence stood."