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 men 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.
"Carlos was able to have a decent pace the last couple of laps. It means it's not that we are losing the tyres. Charles from lap one was complaining about the balance on the first stint, and from the lap one to the last one."
Vasseur also downplayed the suggestion that the car suffers more when running in dirty air.
"It's true also that in quali you are in free air, and races you are not for example.
"Could be an option, but if you have a look at the second stint of Carlos he was also free air, and it was a disaster."
Vasseur conceded that the team had to put more development focus on addressing the consistency problem.
"Perhaps we can steer a little bit of the development on the consistency," he said. "And to have something a bit more easy to drive and so that we can steer a bit. It's the direction that we took the last couple of months or weeks.
"And I think that we are a bit more consistent than we were at one stage. The issue is more than on the chassis side, the issue is more from stint to stint because if you had something like this you could say that it's always there.
"But we had stints in free air that were very difficult, like Carlos's second one."
