Vettel qualified in second, a place behind Leclerc - who had his third pole of the season - but the four-time world champion could only manage a fourth-place finish in Belgium.
"It wasn't an easy race in terms of tyre management, and that's really where Charles showed his class," Windsor said on his YouTube channel. "We have to put (Charles' first win) into context, with Michael Schumacher winning his first race at Spa - and of course - Jim Clark, too.
"There was a big double whammy for Leclerc; with the way he beat his team-mate. Vettel had a very quick car and we shouldn't play too lightly with this - we have got to bear in mind Vettel's experience, Vettel's knowledge of F1 in general with four world championships.
"Charles made him look like a number two driver... yet again!"
Vettel was suffering with tyre degradation, something he confirmed he'll look at before Ferrari's home race this weekend, but Windsor believes it's down to the drivers' style rather than a technical issue.
"The news is, Sebastian, it's down to the way you drive, and the way Charles drives. If Seb is going to take the fight to Charles in those areas, he's going to have to improve," Windsor continued.