Mercedes driver Lewis Hamilton steers his auto during the first practice session for the Bahrain Grand Prix.
On Friday, Vettel finished the second session just 0.041 seconds clear of Hamilton's Mercedes teammate Valtteri Bottas under the lights of the Sakhir circuit.
Kimi Raikkonen, the 2007 champion, was fourth quickest in the second Ferrari ahead of Mercedes' three-time champion Lewis Hamilton, who shares the title race lead with Vettel after two races so far this season.
"I'm thrilled to be making a one-off return to Formula One, and I couldn't think of a better place to make that return than my adopted home grand prix of Monaco", Button, 37, said.
Mercedes' Lewis Hamilton believes that rivals Ferrari has the quickest auto in race specification, following Friday's pair of practice sessions for the Bahrain Grand Prix.
"We had a power unit issue with Kimi - it was quite obvious", said Ferrari technical director Mattia Binotto. Mercedes' Lewis Hamilton was fifth. In race trim they seem a couple of tenths quicker, so I'm expecting a great battle tomorrow and then again on Sunday.
Meanwhile, his Ferrari was lifted off the track by a crane, with the team saying the problem was overheating in the turbo area. The problem on Vettel's auto appears to be completely unrelated and he suspects it was down to a software glitch.
"It was not our best day for us and we still need to do better".
Vettel's fastest lap was more than four-tenths of a second slower than 2016 world champion Nico Rosberg's fastest at last year's event. I'm sure we can fix it. Hamilton came across the Hulkenberg at Turn 10 and ran wide outbraking himself to avoid the Renault, gesturing at the German before saying on team radio: "These drivers are so risky, man".
Teen driver Lance Stroll was sixth quickest for Williams, but the 18-year-old Canadian lost grip late in the session.
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Hamilton was back to winning ways in Race 2 of the year, holding off Vettel with a display of dominant driving from the front in testing conditions.
Hamilton and Vettel are level on points, the former drove to victory at the Chinese Grand Prix, Vettel captured Ferrari's first race victory since 2015 in the season-opening Australian Grand Prix.
In the first practice, Hamilton saved his pace and did not push for a leading time.
However, temperatures will be considerably lower for Sunday's race, which starts in the evening and finishes under floodlights.
Some of you probably thought I was going insane last week when I tipped Max Verstappen to win the Chinese Grand Prix but his third place finish showed he is one of the best when it comes to navigating that erratic wet weather.
"It was incredibly hot early on, with some of the very toughest conditions, both physically, and in terms of working with the tyres", Hamilton said afterwards.
Raikkonen's session ended early after his Ferrari sustained engine damage before he completed a timed lap.
"Walking through the desert in 37C heat".
Raikkonen's solitary walk through the arid landscape in overalls and helmet, before being collected by a scooter, was an arresting sight - if also typical of a driver with "Iceman" tattooed on his left arm.
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