Vunipola was initially sent to the sin-bin for a dangerous tackle on Andrew Porter during Saturday's 29-10 defeat in Dublin, but the offence was upgraded to a red by the bunker review system.
An independent disciplinary hearing reduced the entry-level ban of six weeks to three because of mitigating factors and Vunipola, the only No. 8 in England's World Cup squad, will have another week removed if he attends World Rugby's 'tackle school'.
The 30-year-old Saracens back-rower is now ruled out of England's final warm-up fixture against Fiji at Twickenham on Saturday and their key tournament opener against Argentina in Marseille on September 9th.
Wednesday's announcement by the Six Nations, which oversees disciplinary matters for some tournament warm-up games, comes the morning after Farrell was given a four-match ban for his dangerous tackle on Wales flanker Taine Basham on August 12th.
Vunipola will be available for the Pool D match against Japan in Nice on September 17th, but fly-half Farrell will not be able to be selected by England coach Steve Borthwick until his side's final group games in France against Chile and Samoa.
During a video hearing held late on Tuesday, Vunipola admitted his shoulder-led tackle to the head of Porter was a red card offence.
That in turn triggered an automatic six-game ban.
But a three-strong panel including former Scotland coach Frank Hadden, found mitigation in Vunipola's previously clean record, his immediate apology and the lack of aggravating factors, with the result that the ban was effectively cut in half.
"The player admitted that he had committed an act of foul play worthy of a red card," said a statement issued on behalf of the disciplinary committee.
"On that basis, the committee applied World Rugby's mandatory minimum mid-range entry point for foul play resulting in contact with the head. This resulted in a starting point of a six week suspension.
"Having acknowledged mitigating factors including his exemplary previous record, immediate apology and remorse, and noted the absence of any aggravating factors, the committee reduced the six week entry point by three weeks."
'Tackle school', officially known as the coaching intervention programme, can only be attended by a player once in a career and is designed to iron out the faults that lead to dangerous tackles.