England also need to replace Croft, with the leading back-row options being Joe Launchbury, James Haskell and Carl Fearns.
Lancaster has already made one change to the original elite squad he announced in July, replacing retired prop Matt Stevens with Bath's David Wilson.
Wilson's selection opened the way for Vunipola to be named in the England Saxons - but the 21-year-old is set to be promoted again after an impressive start to his first full-season in the Premiership.
The 20-stone loose-head 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.
Vunipola missed the whole of last season through injury after moving from Bristol to Saracens, but he has already made a big impression this year.