The car has regularly been fast over one lap, which was evident once again at the Spanish GP, where Carlos Sainz qualified second.
However, the team continues to struggle in races, with the car behaving differently from stint to stint.
In Barcelona, Sainz slipped down to fifth place at the flag, while from a pitlane start following his qualifying issues, Charles Leclerc could not rise higher than 11th.
Both drivers had to deal with difficult cars.
"I think the main issue for us is not the potential on the lap, or this type of corner or this other," said Vasseur. "The main issue is the consistency.
"Charles's car for example, between the first and third stint with the same compound, the first one the balance was out of the place, and the last one was okay-ish.
"And Carlos he did a decent first stint, a good last stint, and in the middle he lost 15 or 20 seconds on the competitors."
He added: "It's very difficult to understand and to fix it, because it's not always the same, not always the same problem."
Vasseur said it wasn't a matter of tyre degradation in Spain.
"I don't think that it was tyre deg. It could become tyre deg if you push more. But it's not the main issue.
Source: Autosport