Vettel topped the morning session for the second consecutive day but saw his lap time eclipsed by McLaren's Sergio Perez.
"We had some issues with reliability today and we couldn’t do as many laps as we wanted, but I think it’s better that this kind of thing happens today rather than in Australia," he said after completing 84 laps at the Circuit de Catalunya near Barcelona.
"It was nothing disastrous, just small things that hampered us today. We stopped the car once as we saw a small loss of pressure. The second stop towards the end of the day caused a red flag and it simply took too much time to turn everything around, so we couldn’t get out again."
Mark Webber will run for Red Bull for the remainder of the test.