Leclerc clocked one minute 42.519 seconds. Vettel was 0.748 seconds slower.
It was time Ferrari had clinched the first two place in a race since the Bahrain Grand Prix in March.
Drivers' championship leader Lewis Hamilton was third just ahead of his Mercedes teammate Valtteri Bottas.
"We were struggling a little bit for race pace and we need to work on that but I am sure we will be strong on Sunday," said Leclerc.
Hamilton has won eight of this year's 12 races and leads Bottas in the championship by 62 points. Leclerc has yet to claim a Grand Prix victory.