Leclerc was penalised for impeding Lando Norris in the tunnel in Q3 after his team failed to warn him early enough about the fast-approaching McLaren driver.
The penalty dropped Leclerc from third on the grid to sixth, making his life much harder for Sunday's race, in which he eventually finished in the same position.
Leclerc, who has regularly had bad luck in Monaco, tried to be positive after a disappointing afternoon.
"Yesterday I got the information too late and that of course put us a little bit on the back foot for today," he said when asked about the penalty by Motorsport.com.
"Of course it's frustrating. Any weekend like this one anywhere it's frustrating, at home it's even more frustrating. But on the other hand that's where we need to keep our head up.
"The season is long, it's not the only race on the calendar even though it probably means a bit more than the others. The points are the same, so we need to restart again in Barcelona, hopefully having a great weekend that time."
Vasseur said that the team would try to learn from the communication mistake and ensure that there is no repeat.
"He's been a bit frustrated in Monaco for a couple of years now and for sure yesterday was tough," said the Frenchman.
"It was tough first to lose the pole position for one tenth or something like this, to lose the first row for two hundredths, and then the call.
"What can I do else than apologise on behalf of the team and to understand how we can do a better job, and to improve the communication between the pitwall and Charles in this condition, but on both sides.
"I think it's also due to the circumstances where its happened, into the tunnel and so on. But okay, we have to avoid to try to find excuses because it's the worst way if you want to improve, we just have to correct."
Expanding on what went wrong, Vasseur said: "Under the tunnel, Charles was not able to see in the mirror where was Norris, and if you tell him after the tunnel he can see where is Norris, and avoid to be on the line. Also the line under the tunnel is not very clear.
"But we made a mistake yesterday clearly and this cost us a lot because I think starting from P3 it could have been another race, but we have to learn from it."
He added: "Honestly, I'm not looking trying to find an excuse or something like this, we have to review completely the situation tomorrow.
"I didn't want to do it yesterday evening because we are focused on the race. But tomorrow we will have a look on this."
