The duo are set to make their England debuts in Saturday's international after being included in the 23-man squad.
Launchbury, who initially only linked up with the squad on Monday as injury cover for Tom Palmer, has been preferred to Saracens lock Mouritz Botha and Harlequins' George Robson.
Vunipola's inclusion as one of four props in England's match-day squad was well sign-posted after an impressive start to the Aviva Premiership season with Saracens.
England head coach Stuart Lancaster will confirm his starting XV and eight-man bench for the Fiji clash on Thursday morning.
That starting line-up is set to include the uncapped Leicester hooker Tom Youngs, who is in line to make his Test debut in place of the injured Dylan Hartley.
Like Vunipola, Youngs was always going to win a place in the 23-man squad by virtue of the fact that he and David Paice were the only two fit hookers available.
Launchbury is likely to be picked on the bench as back-up for his Wasps club-mate Tom Palmer and England's lineout general Geoff Parling.
Leicester's Thomas Waldrom is in line to start at number eight after Gloucester's Ben Morgan was among 10 players released back to their clubs on Tuesday night.
James Haskell was also released, with Tom Wood returning to an England match-day squad for the first time since the Rugby World Cup pool match against Romania in Dunedin nearly 14 months ago.
Wood is competing with Exeter's Tom Johnson for the blindside flanker berth.
Anthony Allen, Freddie Burns, Alex Corbisiero, Lee Dickson, Phil Dowson and Billy Twelvetrees have also been released to play for their clubs this weekend.
Lancaster said: "We have had a good two days back at Pennyhill Park and the players have been very focused on delivering against Fiji.
"Saturday is a great opportunity for us to put a marker down and deliver a quality performance.
"It's great to have young players such as Mako Vunipola and Joe Launchbury get their opportunities in the match-day 23 and we are excited about the potential of the squad."
England's 23-man squad for Saturday's QBE autumn international against Fiji at Twickenham (kick-off 2.30pm):
Forwards: D Cole (Leicester Tigers), T Johnson (Exeter Chiefs), J Launchbury (London Wasps), J Marler (Harlequins), T Palmer (London Wasps), D Paice (London Irish), G Parling (Leicester Tigers), C Robshaw (Harlequins, capt), M Vunipola (Saracens), T Waldrom (Leicester Tigers), D Wilson (Bath Rugby), T Wood (Northampton Saints), T Youngs (Leicester Tigers)
Backs: B Barritt (Saracens), M Brown (Harlequins), D Care (Harlequins), O Farrell (Saracens), T Flood (Leicester Tigers), A Goode (Saracens), U Monye (Harlequins), C Sharples (Gloucester Rugby), M Tuilagi (Leicester Tigers), B Youngs (Leicester Tigers)
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