The four-time world champion, also quickest on the opening day, set a new overall benchmark of 1m 20.984s to finish almost one second clear of the chasing pack, despite a rain storm bringing a slightly early close to proceedings.
“It’s too soon to give a verdict on the true level of this car, especially as we can’t make a comparison with the other teams, who can be running different programmes," said Vettel. "However, I’d say we have a good basis to work from.
"We did a lot of running, even in the rain, when we tried the new intermediate tyres, which worked well, getting up to temperature despite the cold: that’s a definite step forward. The applause in the garage at the end of the session? Well, everyone was happy we’d even managed to run in the wet with nothing bad happening. The boys have got enough to do in the garage as it is…”.
Sauber’s Felipe Nasr and Toro Rosso’s Max Verstappen - both entering their first season in Formula One racing - were among the others who ventured out in the wet conditions.
It was Nasr who ultimately got closest to Vettel, putting Sauber second with a 1m 21.867s. That left him half a second clear of Williams’ Valtteri Bottas and the Mercedes of 2014 world champion Lewis Hamilton.
"It was a great day," Nasr reflected. "I was able to complete a lot of laps, and I was happy with everything I saw. I was getting used to the systems, the procedures and the characteristics of the car. Now we can start working on the details."
Hamilton was able to clock up 91 laps, the most of any driver, as Mercedes once again demonstrated impressive reliability - although a water leak from a failed component brought his day to a slightly premature finish.
"That was a satisfying day," said the reigning champion. "We did a lot of laps and I got a feel for the car quite quickly. We're in the first days of testing so it never feels that great as the car is at the worst it will be all season. But it feels the same as it did last year which is a good thing. A little more downforce maybe but otherwise it handles exactly the same.
"The balance obviously wasn't perfect as we're not working on set-up yet, so it could be a lot better. But the priority right now is to put in the laps, not set fast times, and there were lots of positives today. We got good mileage and hopefully we'll get even more in the next two days."
Verstappen was fifth fastest, 1.5s down on Hamilton, while Lotus’s Pastor Maldonado and McLaren’s Jenson Button rounded out the top seven.
"In the morning we had some difficulties in warming up the brakes, but we fixed that very quickly," Verstappen said. "After that, we managed to do some long runs that helped me to learn about the new car straight away. In the afternoon it rained a bit, but it was useful for me to experience the intermediate tyres. It feels good to finally start working for the upcoming season."
Lotus had initially played down their chances of running before the third day of the test as they finalised the build programme of their E23 Hybrid. They were able to join the fray early in the morning, however, with Maldonado eventually racking up 41 laps.
“It was fantastic to get behind the wheel for the first ever lap of this exciting new car," said Maldonado. "Even though it’s early days we achieved a lot today. It was already exciting to feel the potential of the car and I can’t wait to get back in the car tomorrow. The car certainly looks fantastic and I know we have a lot of potential for the season ahead.”
McLaren’s progress with their new Honda-powered car was more limited, as a recurring problem confined Button to the garage for much of the day. The team did emerge in the final minutes, but only for a sedate out-lap. Button therefore ended the day with six laps - matching what 2015 team mate Fernando Alonso managed on day one - and an unrepresentative best time of 1m 54.655s, more than 30s down on Vettel.
“Today was another tricky day, but we feel like we’ve overcome the main issue that affected our running during the first two days of the test," said Yasuhisa Arai, Honda's chief motorsport officer. “For tomorrow, we’re hopeful that we’ve moved on, and that we can get some more laps completed.”
Daniil Kvyat was the eighth and final runner for Red Bull, although the Russian did not set a time on his first day in the RB11 after damaging the front wing in an early spin. With no replacement, Kvyat had the unusual task of completing several installation runs with no front wing.
"Sometimes mistakes happen and there's nothing you can do about them," Kvyat said. "After that we tried to maximise our day, not having spare parts to replace the wing. We still maximised our running with what we had and hopefully we can recover from that."
The only team absent from the timesheet were Force India, who have opted to skip the first test completely whilst they develop their new car.
Testing continues at Jerez on Tuesday.
Unofficial Monday testing times from Jerez:
1. Sebastian Vettel, Ferrari, 1m 20.984s, 88 laps
2. Felipe Nasr, Sauber, 1m 21.867s, 89 laps
3. Valtteri Bottas, Williams, 1m 22.319s, 61 laps
4. Lewis Hamilton, Mercedes, 1m 22.490s, 91 laps
5. Max Verstappen, Toro Rosso, 1m 24.167s, 73 laps
6. Pastor Maldonado, Lotus, 1m 25.802s, 41 laps
7. Jenson Button, McLaren, 1m 54.655s, 6 laps
8. Daniil Kvyat, Red Bull, No time, 18 laps