The German, in a 2015 Ferrari for the first time after his move from Red Bull, headed Sauber's Marcus Ericsson.
The new Mercedes made an ominous debut, with Nico Rosberg third fastest after completing a remarkable 157 laps.
Fernando Alonso managed only six laps as McLaren began their engine partnership with Honda.
Alonso was 18 seconds off the pace as the team struggled to get the car and engine running together - just as they had when they tried to run the engine in a 2014 car at a post-season test in Abu Dhabi in November.
McLaren played down the situation, saying the car had been kept in the pits by "a few niggly problems".
Alonso seemed unperturbed, writing on his Twitter account: "First laps. So proud of my team. Happy."
The two-time champion hands over to Jenson Button on Monday as the two McLaren drivers alternate throughout the four days of the test.
Meanwhile, McLaren chief executive Ron Dennis added that the car's livery - standard silver and black for now - would change at some point during the year, although he would not say when.
Vettel's fastest time was a one minute 22.620 seconds lap and his position at the top of the time sheets was a mirror of the same test last year, when his new team-mate Kimi Raikkonen set the pace for Ferrari.
The Italian team went on to have their worst season for more than 20 years, precipitating a series of sackings of top managers.
The team lost its president, two team principals, and several senior design engineers by the end of 2014 and are not expecting to be competitive at the start of this year.
Ericsson's pace for the struggling Sauber team underlined the relative meaninglessness of headline times in testing, when it is impossible to know what specification the cars are running in.
But there was no escaping the fact that Mercedes had a successful first day.
Rosberg completed more than twice as many laps as any other driver and was able to lap at will in the one minute 23 second bracket.
World champion Lewis Hamilton was not allowed to drive as each team is only allowed to run one car per day.
The team that dominated last season, winning 16 of the 19 races, is again a strong favourite this year.
Red Bull's Daniel Ricciardo was fourth fastest, 0.232secs behind Rosberg, but Red Bull chief technical officer Adrian Newey said he did not expect the team to be able to challenge Mercedes for the championship this season.
"It was a decent first day, usual new car niggles, a battery issue and an engine problem, but nothing major so far," Newey said.
He said the Red Bull, running in a temporary camouflage livery, was "hopefully a decent step forward from last year", but he added that a title challenge would be "extremely difficult, in truth.
"The Mercedes power advantage was about 10% last year and that is not something Renault can close in one season. we are still going to be on the back foot but we will keep pushing."
Williams's Valtteri Bottas was fifth fastest, completing the second highest number of laps in his Mercedes-powered car, ahead of Toro Rosso novice Carlos Sainz Jr and Alonso.
Lotus are also due at the test but did not run on Sunday as their new car was being flown over to Spain. The team hope to run on Monday.
Force India are missing the test because their 2015 car is not finished.
The test ends on 4 February.
1 Sebastian Vettel (Ger) Ferrari 1:22.620
2 Marcus Ericsson (Swe) Sauber-Ferrari 1:22.777
3 Nico Rosberg (Ger) Mercedes 1:23.106
4 Daniel Ricciardo (Aus) Red Bull-Renault 1:23.338
5 Valtteri Bottas (Fin) Williams-Mercedes 1:23.906
6 Carlos Sainz Jr (Spa) Toro Rosso-Renault 1:25.327
7 Fernando Alonso (Spa) McLaren-Honda 1:40.738
Ferrari's Sebastian Vettel set the pace on the first day of pre-season testing as Mercedes impressed and the new McLaren-Honda struggled.