The experienced driver sealed a two-year deal with Sauber, which changed to Alfa Romeo for this season.
The Finn currently sits 8th in the standings and has done an impressive job for his new team, who are placed 7th in the constructors.
Vasseur told Autosport: "The advantage with Kimi is before he signed last year we had hours of discussions, about the team, the positive and negative points, where we could improve, where we have to improve.
"We were very open about what is the real situation of the company.
"And I think now he is not surprised when we have issues because we knew that before.
The 39-year-old has enjoyed a successful career in the sport, which was topped off by his Championship victory back in 2007, and has won a total of 21 races.
"The most important for me was to make sure he was committed," said Vasseur.
"Committed means he will be present at the factory, he will push the guys, he will be demanding.”