Vunipola (pictured) is one of the most exciting young players in the Premiership and he has been fast-tracked into England's Six Nations plans. The dynamic 20-year-old will join brother Mako at Saracens, who is one of 12 players to have agreed contract extensions with the Men In Black.
While the loss of Vunipola will hit Wasps hard, just as director of rugby Dai Young looks to have finally got the club moving in the right direction, Johnston's departure is a huge blow to Harlequins.
The Samoa international was the cornerstone of Harlequins' title triumph last season and he has developed over the last year into one of the most dominant tight-head props in the Premiership.
England internationals Richard Wigglesworth and David Strettle have also agreed new deals with Saracens along with Saxons squad member Will Fraser. Eoin Sherriff, Ben Spencer, Nils Mordt, Duncan Taylor, Ben Ransom and James Short have also extended their stays at the club.
Wasps had been led to believe Vunipola would stay at Adams Park, where he is one of a number of young players who have helped lift the club from the brink of relegation last season to fourth place in the Premiership.
Club officials are understood to be furious that he has decided to leave and have promised a full reaction on Tuesday.
In the meantime, a Wasps statement said: "The club has been informed this afternoon that Billy Vunipola has decided to leave at the end of the season. We are incredibly disappointed that he has chosen to leave and we will make a full comment on his departure tomorrow."
- Sports & Recreation