The 26-year-old quadruple champion has suffered a series of setbacks since he arrived at the Circuit de Catalunya with a different chassis to the one he had raced in the first four rounds of the season.
A problem with the wiring loom limited his track time to four laps in Friday practice and the German pulled over in the final phase of Saturday's qualifying with gearbox problems that left him 10th on the provisional grid.
Gearboxes must last six successive races and any unscheduled change incurs a five place penalty.
"I left the pits and...I lost drive in second gear. I went to third, decided to keep going because its possible to do a lap without second gear around here but by turn one I got stuck in fifth gear and couldn't go up or down," he said.
The champion, who won the last nine races of 2013 but has so far made just one podium appearance this season with a third place in Malaysia, recognised after qualifying that Sunday's race would be difficult.
"It's obviously tough because you don't get more information on the car, get into the rhythm. We missed more or less both practice sessions yesterday," he said.
Vettel said Friday's problems came down to "a very small cable (that) got caught in the wrong place which is impossible to see when you put the car together.
"Obviously now we know where it belongs," he said. "Up to that stage you could say we've just been lucky because it never got caught."
Vettel has struggled to get the feeling he wants from the new V6 turbo powered car, and its complex energy recovery systems.
In the meantime, his new Australian team mate Daniel Ricciardo has been going from strength to strength - a marked change from last year where his predecessor and compatriot Mark Webber seemed to attract all the bad luck in the team.
Ricciardo qualified third on Saturday, the fourth time in five races that he has been quicker than Vettel on a Saturday afternoon.
"It's not a nightmare. We were able to learn a lot of things," said Vettel of his season so far.
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