The Gloucester back row, who made a half century of appearances for the Scarlets between 2009 and 2012, has come on as a replacement in all three of England's RBS 6 Nations games so far this term but was widely expected to start against Wales once Vunipola was ruled out with an ankle problem.
Morgan will face a host of former team-mates in the likes of Rhys Priestland and Jonathan Davies and co in the Round 4 encounter in London as he wins his 19th cap for an England side looking to secure a first Triple Crown in 11 years and effectively end Wales' Championship prospects in the process.
"Ben has made an impact off the bench in the previous games. He has been pushing hard for the No 8 shirt and now gets his opportunity so we are looking forward to seeing him from the start," said England head coach Stuart Lancaster.
"Tom Johnson has been outstanding for Exeter Chiefs and gives us options off the bench as he has played in all three back row positions for his club. We can't wait to play at home again.
"The players have said that the atmosphere at Twickenham for the Ireland game was the best they had experienced and I am sure it will be the same again on Sunday. It definitely gives us an extra lift and that's what we want against another experienced, high quality side, who are looking to defend their title."
Exeter flanker Johnson is the only newcomer in the matchday 23 after he stepped up to take Morgan's position on the bench. Lions centre Manu Tuilagi and London Irish wing Marland Yarde were both fit enough to be considered for selection having appeared for their clubs at the weekend, but Lancaster opted against including them when he named his matchday squad on Wednesday.
British & Irish Lions tourists Owen Farrell and Billy Twelvetrees start for the English, with fellow Lion Tom Youngs among the replacements. Inform Mike Brown is named at full back after Wales head coach Warren Gatland described him as the 'player of the tournament so far', while Harlequins team-mates Danny Care and Chris Robshaw will be looking to keep their impressive displays going for another two weeks at least.
Jonny May and Jack Nowell continue on the wings, with Luther Burrell joining Twelvetrees in midfield after two tries in his first three Test appearances.
Captain Robshaw and Morgan are accompanied by Tom Wood in the back row, while Joe Launchbury and Courtney Lawes continue their impressive partnership at lock in the absence of Test Lion Geoff Parling.
David Wilson stays at tight-head following Dan Cole's injury setback; Dylan Hartley sees off the challenge of Youngs to remain his country's first-choice hooker; and Joe Marler is again preferred to Mako Vunipola despite the latter's showings for the Lions last summer.
ENGLAND (v Wales)
15 Mike Brown (Harlequins, 24 caps)
14 Jack Nowell (Exeter Chiefs, 3 caps)
13 Luther Burrell (Northampton Saints, 3 caps)
12 Billy Twelvetrees (Gloucester Rugby, 11 caps)
11 Jonny May (Gloucester Rugby, 4 caps)
10 Owen Farrell (Saracens, 22 caps)
9 Danny Care (Harlequins, 45 caps)
1 Joe Marler (Harlequins, 18 caps)
2 Dylan Hartley (Northampton Saints, 53 caps)
3 David Wilson (Bath Rugby, 32 caps)
4 Joe Launchbury (London Wasps, 17 caps)
5 Courtney Lawes (Northampton Saints, 28 caps)
6 Tom Wood (Northampton Saints, 26 caps)
7 Chris Robshaw (Harlequins, capt, 23 caps)
8 Ben Morgan (Gloucester Rugby, 18 caps)
16 Tom Youngs (Leicester Tigers, 15 caps)
17 Mako Vunipola (Saracens, 13 caps)
18 Henry Thomas (Sale Sharks, 4 caps)
19 Dave Attwood (Bath Rugby, 8 caps)
20 Tom Johnson (Exeter Chiefs, 5 caps)
21 Lee Dickson (Northampton Saints, 14 caps)
22 George Ford (Bath Rugby, uncapped)
23 Alex Goode (Saracens, 15 caps)