Vunipola blow for England
Saracens No.8 Vunipola suffered a knee ligament strain during last Saturday's 25-28 loss to Wales.
He had a scan on Monday and has been ruled out for the remainder of the tournament.
Harlequins loose forward Nick Easter has been called up as a permanent replacement for Vunipola.
Easter joined up with the squad on Monday.
"It's unfortunate for Billy, but the scan shows an injury which will likely require up to six weeks recovery," and England coach Stuart Lancaster said.
"He has worked really hard and committed everything to the last three months, but he is a young man with a bright future ahead of him and he will be an integral part of this team for years to come.
"Nick was unlucky to miss out on final selection.
"He is a specialist No.8 and an experienced player and we know he will make a big contribution."