Valtteri Bottas and Lewis Hamilton fought each other for pole position on Saturday with the Finn coming out on top, while Vettel ended up four tenths of a second adrift.
"In Q2 I thought OK, it will be tight', happy to go into Q3 - and then I was very happy with my first lap," said Vettel, who topped both practice sessions on Friday.
And when I got the time, I was a bit down. Four tenths was a lot more than expected given how good I felt.
"On the last run, I tried probably too hard everywhere, we had nothing to lose.
"I tried a bit harder but didn't work, was slightly slower. Being a tenth quicker [wouldn't be] good enough but I tried."
Vettel, who did comfortably outqualify team-mate Kimi Raikkonen, said he wasn't sure why Ferrari's deficit to Mercedes had grown compared to the first two races in Australia and China.
Read the full article on eurosport.co.uk: Vettel 'down' over gap to Mercedes in qualifying