The world champion missed out on a Q3 place by 0.012 seconds at Spa, and said nothing in particular had been wrong.
"I was pretty happy with my lap, it was just not quick enough unfortunately," he said.
"I missed the last part of qualifying by such a little amount and that's a shame, but that's racing. It's frustrating but it could be the other way around and you are in and somebody else is out.
"But obviously there are no points today and the race is tomorrow."
He admitted his disappointing result came as a surprise as he had been comfortable with the car in practice.
"I was quite surprised that this morning it felt pretty alright and this afternoon it just didn't come together," Vettel said.
"The lap was clean, there were no mistakes, but it wasn't quick enough."
His team-mate Mark Webber had similar sentiments after taking seventh. The Australian will lose five places due to a gearbox-change penalty.
"I'm disappointed. I think that we would like to have been further up the grid, no question about it," said Webber.
"Not quick enough for us to challenge for the front row, and that obviously knocks the penalty around a little bit more. I would've liked to have been further up to take the sting off the penalty.
"I was pretty happy with my lap. The maximum was P5 if I'd got the very most out of it.
"I had some very good sections on the lap and then some other sections where I would've liked to have done a little bit better.
"But I got the most out of what we had and at the moment that's not enough."