Last season, Leclerc led the world champions with 71 points after three races and two victories.
This year, after the third round, he is 10th, and his only score in Saudi Arabia was seventh, where he had to recover from a penalty.
He was stopped in Bahrain with an engine problem and in Australia he retired after making contact with Lance Stroll on the first lap.
Vasseur insists that while his frustration is understandable, Leclerc remains fully behind the team.
“I have absolutely no doubts about Charles’ motives,” he said.
“For sure, the start of the season has not been ideal at all. We had a DNF in Bahrain, then a penalty in Jeddah and a DNF in Melbourne.
“It wasn’t the plan at all. But the momentum was there. You know I have a great relationship with Charles. It doesn’t affect the mood.
“But when you jump to Charles or Carlos or any other driver in the TV pen 30 seconds after the race, they’re definitely not happy.
“I would be very upset if they were relaxed and happy about the situation. The most important thing for me is to keep everyone motivated, working in the same direction, they are supporting the team. Carlos (last week) In the simulator, Charles (this week) will be there.
“Motivation is not an issue at all. The result is definitely not as expected. But we all know that.”
There was some tension in the training camp following qualifying in Australia, with Leclerc blaming a lack of communication after he had to pass Tour driver Sainz on the final lap of Q3 for seventh.
Vasseur said he spoke with both men about the disappointing session and emphasized that their frustrations were positive.
“As you can imagine, I talked to them a lot,” said the Frenchman. “Like I said, when you bash the drivers in the TV column, they’re not doing well, they’re not doing well, they’re not doing well or they’re not racing well, they’re always frustrated.
“That frustration is a positive for me because if it wasn’t for that, I would be disappointed.
“And then the story about the location of the car, it was a very late decision, more opportunistic than something like this, and it’s not easy to do. I take the responsibility for this, not the driver.
“But we had long discussions about this and other qualifiers because I think it’s more than that and we haven’t done a good job as a team in terms of preparation and so on and we have the potential to do better .
“I think Charles felt it too, the frustration to the point.”
Asked whether there had been interference from above given the poor start to the season, Vasseur insisted the team had a good relationship with Ferrari’s top management, chairman John Elkann and chief executive Benedetto Vigna.
“For the level of results we’ve achieved, the atmosphere in the team is very good,” he said. “I think everyone is very motivated, very focused. The drivers are very supportive of us.
“The atmosphere in the team is very good. If the question is about the relationship with John or Benedetto, believe me, it is also very positive.
“Of course we didn’t get the results we expected, but we are all working together to improve the situation.”