The 21-year-old Vass had been one of Northampton's bright prospects as cover for England star Matt Dawson, but following the Saints' recent recruitment of All Black scrum-half Mark Robinson, he was deemed surplus to requirements.
At Stade Français Vass will also find himself behind one of the game's top names – veteran France captain Fabien Galthié – but may well get a run in the side during the World Cup.
Stade Français have also swooped for Bristol's Argentina scrum-half Agustin Pichot, giving them a wealth of depth in the position as they look ahead to the possible retirement of the 34-year-old Galthié, who is still playing some great rugby.
This season has seen Stade Français fail to recover properly from the sale of Canadian back-up No.9 Morgan Williams to Saracens – Jerome Fillol coming in from Toulouse to do a job during Galthié's lengthy shoulder injury, although he was this week sold to Perpignan.
Vass joins a Stade Français influx including Italian brothers Mirco and Mauro Bergamasco, as well as Colomiers' Test fly-half David Skrela.
Meanwhile, Stade Français prop Laurent Emmanuelli has joined Montferrand on a two-year deal.
By Mark Smith