The Monegasque driver started from pole and, having lost out early on to both Lando Norris and Lewis Hamilton, was the only driver in the field to run a one-stop strategy.
He eventually slipped back to sixth place, although that became academic when he was disqualified for a plank infringement.
Meanwhile, from fourth on the grid, his team-mate Carlos Sainz followed the rest of the field in stopping twice on his way to an initial fourth place in sight of Norris. That became third when runner-up Hamilton was also disqualified.
Speaking before the disqualifications changed the final results, Vasseur conceded that the team had got it wrong with Leclerc.
"Clearly we have mixed feelings," he said when asked about the race by Motorsport.com. "Because on one side of the garage with Carlos, we did pretty well. Starting P4, finishing P4, we finished two or three seconds behind Norris, and I think everything went well.
"With Charles, who was 10 seconds in front of Carlos after 12 laps, we committed for one stop, and it was not the good choice, it's obvious.
"Probably the issue is that we didn't have a clear picture about this before the race, we were a bit hesitating, and he was a bit hesitating into the first stint, pushing or not pushing. And we made a mistake.
"It was not very clear before the race, as you can imagine. We had the two options. In terms of numbers, it was very, very close.
"I think where we made the mistake is that we anticipated that the field will be 50-50. And it was not at all."
Vasseur admitted that the team had expected others to pit only once.
"I think that Hamilton was not far away to do one stop, [George] Russell, I think he went for one stop clearly, but as he was the last of the group, it's much easier to change if you are last and you are in the same strategy as the others. It's better to change and to take risks."
Vasseur also stressed that other drivers pitting twice actually made life harder for Leclerc.
"It's not just a matter of tenths of seconds on the tyres, it's a matter of what the others are doing also," he said. "Because what was even more difficult with Charles was that everybody committed for the two stops.
"It means that you have much more traffic around you, because it's not that everybody's doing the same race as you, and you have a clean race, as you are not in the sequence, the guys are overtaking you one or two times.
"And each time that someone is overtaking you, you are losing two seconds. It means that when you have these two times, four guys, you are losing eight times 1.5 seconds, it's 12 seconds for the traffic, and the picture of what users are doing. But it's a mistake."
He added: "I think the main issue was that we were a bit in-between. We moved quickly on the two stops with Carlos, because in the fight with Max [Verstappen], I think he had a bit of deg, and we decided to commit quite early on the two stops. Charles, it was less obvious."
Vasseur insisted that he didn't want to speculate about what Leclerc might have achieved with the same strategy as his team-mate.
"I don't want to do it," he said. "Because if, if, if, if, if, first the French team in rugby would be in the [World Cup] final, and also the English team! And we would have won the World Cup in football two years ago, and so and so.
"For sure you can redo the race and imagine that he was six or seven seconds ahead of Sainz, and Leclerc would have done this or this.
"But I think it's not the right approach, we have to be focused on the mistake, to try to understand why you did the choice, because we were convinced that it was the good one, with the elements that we had at this stage.
"It means that the numbers that we had on the pitwall, and at the factory at this stage of the race, were not good ones."
