Vunipola injury opens England door for Johnson
England's Ben Morgan (L) evades a tackle from Ireland centre Brian O'Driscoll (R) during the Six Nations international rugby union match between England and Ireland at Twickenham, west London, on February 22, 2014 - by Glyn Kirk
Johnson was included in Stuart Lancaster's 23-man matchday squad for the title showdown with the defending champions and will feature among the replacements when the team is announced on Friday.
Number eight Vunipola was ruled out of the remainder of the Six Nations with an ankle injury sustained in the 13-10 victory over Ireland last month.
"Tom Johnson has trained with us throughout the Six Nations and we are looking forward to his contribution," said Lancaster.
"The performance against Ireland has given us a lot of confidence going into this game against a high quality Welsh team. We feel it's right to give the bulk of that 23 another opportunity."
Leicester back Manu Tuilagi, out for six months with a chest injury, will continue to train with England but has not been considered for selection.
England's matchday squad of 23 to face Wales at Twickenham in the Six Nations on Sunday:
Forwards (13): Dave Attwood (Bath), Dylan Hartley (Northampton), Tom Johnson (Exeter), Joe Launchbury (London Wasps), Courtney Lawes (Northampton), Joe Marler (Harlequins), Ben Morgan (Gloucester), Chris Robshaw (Harlequins), Henry Thomas (Sale), Mako Vunipola (Saracens), David Wilson (Bath), Tom Wood (Northampton), Tom Youngs (Leicester).
Backs (10): Mike Brown (Harlequins), Luther Burrell (Northampton), Danny Care (Harlequins), Lee Dickson (Northampton), Owen Farrell (Saracens), George Ford (Bath), Alex Goode (Saracens), Jonny May (Gloucester), Jack Nowell (Exeter), Billy Twelvetrees (Gloucester).