England coach Stuart Lancaster has made four injury-enforced changes to the elite squad he originally named in July for the Tests against Fiji, Australia, South Africa and New Zealand.
Saracens prop Vunipola, whose father played at two World Cups for Tonga, and Leicester hooker Young are covering for Alex Corbisiero and Rob Webber.
Harlequins wing Ugo Monye has also been promoted to fill the back three berth left vacant when Ben Foden damaged ankle ligaments playing for Northampton.
James Haskell, who returned to the England team for the drawn third Test against South Africa in Port Elizabeth on the summer tour, has been promoted from the Saxons to replace Tom Croft.
Lancaster had already made one change to the original squad, replacing retired prop Matt Stevens with Bath's David Wilson.
Vunipola was drafted into the Saxons as Wilson's replacement but the 21-year-old has now been fast-tracked into the senior squad after an impressive start to the season with Saracens.
The 20-stone loosehead prop was born in New Zealand. His father, Fe'ao Vunipola, won 32 caps for Tonga and played at the 1995 and 1999 World Cups.
Although he qualifies for Wales after the family relocated when Fe'ao signed for Pontypool in 1998, Vunipola has played for England Under-18s and the Under-20s.
Youngs, the elder brother of scrum-half Ben Youngs, has only established his place in the Leicester team this season after being converted from a hard-running centre.
The 25-year-old toured South Africa with England in the summer and he will be Dylan Hartley's understudy for the autumn Tests, following Lee Mears' international retirement.
Monye, who won his last England cap against Scotland in 2010, has returned to form with a bang this season, scoring four tries in five Premiership games for Harlequins.
Lancaster said: "As a coaching team we can't wait to go into camp and get the guys focused on what will be a massive challenge in the QBE Internationals - Fiji and then the three major southern hemisphere teams in successive weeks.
"We have been impressed with some of the rugby we have seen in the Aviva Premiership and Heineken Cup and are looking forward to the players bringing that form to Twickenham.
"It's great to be able to give Mako a chance in the squad. He has come through our age grade sides and made a real impact at Saracens.
"Likewise Tom Youngs, who was with us on tour in South Africa, and we know what James Haskell and Ugo Monye are capable of if they get their opportunity."
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