Leclerc won his first ever Formula 1 race after qualifying on pole, with Vettel starting a place behind his Ferrari team-mate, but the four-time world champion could only finish in P4 after a disappointing race.
"For sure, it's a good day for the team after a long drought," Vettel said to RaceFans. "I'm happy for the team, happy for Charles. But on my side, I can't be pleased with how the day went. We started second and finished fourth.
"Obviously the car had more so I can't be too happy with that. I was struggling to feel the grip, to stay on top of the tyres. I was sliding too much so it was quite a tough race for me. And obviously from the point that we came (to the pits) quite early it was clear that we won't be there until the end."
Vettel had problems all race, and he admitted he wasn't sure why he was having the issues: "No. I guess I am sliding more but I don't know why."
Ferrari boss Mattia Binotto, like Vettel, is unsure what happened to the German driver in Belgium: "Certainly Sebastian suffered more from degradation this weekend; both on Friday and then in the race. It's certainly down to the set-up as well.
"Normally Seb is very good in managing tyres, especially in the first laps, to use the tyres later in the stint. So it's something on which we will take care, try to understand and analyse, and certainly something that if we may learn it, will improve ourselves in the future. But so far, no answer."