Less than a mile into Sunday's Styrian Grand Prix, both scarlet cars were effectively eliminated after Leclerc smashed into Vettel on the opening lap.
Leclerc attempted to overtake Vettel at the third corner but instead collided with the German.
The force of the impact ripped off Vettel's rear wing and the four-time world champion was forced to retire.
Leclerc stopped for repairs and attempted to solider on in his wounded car before he too was forced to park up.
It marked the second time in four appearances that the Ferrari drivers have taken each other out of a race, following last year's embarrassing coming together in Brazil.
"What happened today is clearly my fault, and there's nothing else to say," said Leclerc, 22.
"I take full responsibility. I made a mistake and apologising is not enough.
"I was so eager to do well for the team and I thought I might be able to gain three or four places and I went for it. But in fact the opportunity wasn't there.
"We are going through a difficult time and we don't need this. I have apologised to the team and to Seb, who didn't do anything wrong.
"I let everyone down today and I am very sorry."
Team principal Mattia Binotto is under enormous pressure at the Scuderia, with this year's car miles off the pace.
Leclerc started 14th on Sunday, four spots behind Vettel, who is leaving Ferrari at the end of the year after the Italian team chose not to renew his contract.
"It is very painful to see both our cars back in the garage after just a couple of laps," said Binotto.
"It's the worst possible end to a weekend that was already very disappointing."
