The incident occurred against Worcester on 14 September
England flanker James Haskell has received a one-week ban for a head-butt during Wasps' 11-10 loss to Worcester.
A Rugby Football Union disciplinary panel said the incident was "offending of the least serious kind where there are compelling mitigating factors".
Haskell will miss Wasps' Premiership clash with Leicester on Friday.
But RFU disciplinary chief Judge Jeff Blackett criticised referee Martin Fox and citing officer Matt Bayliss for their handling of the incident.
Blackett, who chaired the hearing, felt Fox failed to take action against Worcester duo Tom Wood and Greg Rawlinson, who sparked the incident with their aggression towards Wasps prop Phil Vickery.
Wood, according to Blackett's report, jumped into Vickery with a swinging arm at a ruck, before Rawlinson punched Vickery five times in the head after the former England captain had grabbed Rawlinson's scrum cap.
Had the referee taken action against Wood initially, then none of the rest of the foul play would have followed
Haskell requested Fox intervene, but when the referee did not, the England flanker cleared out Rawlinson by lunging at him head first.
"The case has not been handled well by match officials or the citing officer," said Blackett.
"The referee was very close to the incident and must have had a clear view of what occurred, yet took no or insufficient disciplinary action.
"Had the referee taken action against Wood initially, then none of the rest of the foul play would have followed.
"It is almost beyond comprehension that he then decided to penalise Vickery for the least of the all the acts of foul play."
Blackett also felt the citing officer Bayliss failed to describe the incident well enough in his report.
"Unfortunately, the citing officer also took insufficient action," said Blackett.
"His written observation of what occurred failed to describe adequately or properly the full extent of the various offences committed."
Blackett indicated that Rawlinson should also have been cited for punching, and Wood for illegal clearing out.
"By failing to do so the panel were placed in the invidious position of taking disciplinary action against one person when other similar acts of foul play in the same incident will remain unpunished because the time limit for citing has passed and there are no exceptional circumstances which would allow those limits to be waived or extended," his report said.
The minimum recommended entry-level punishment for a head-butt is six weeks with a maximum sanction of two years, but the panel considered Haskell's an exceptional case with "compelling mitigating factors".
"(Haskell) is not a persistent offender albeit he has two yellow cards on his record," said the RFU panel decision.
There were mistakes made in the process but it was a fair review and outcome
"In this incident he reacted to severe provocation - a fellow player was under sustained attack from a series of punches to the head and, even though he had brought it to the referee's attention, no action was being taken.
"His action was instinctive to protect a team-mate.
"He accepted that he should not have taken this action, but he felt aggrieved that he was the only person who had been cited when there were clearly others who had committed equally serious offences in the same incident."
Wasps director of rugby Ian McGeechan said he was happy with the outcome of the hearing.
"There were mistakes made in the process but it was a fair review and outcome," he said.
Fox, a 46-year-old company director, has served on the RFU's elite referees' list for six years, and is scheduled to take charge of Saturday's Premiership match between Saracens and Northampton.
Meanwhile Haskell's Wasps team-mate Richard Birkett has been reprimanded and warned following his citing for stamping on Worcester's Netani Talei during the same Premiership match on 14 September.