Button’s team-mate Lewis Hamilton was fourth, ahead of the two Lotus drivers, Kimi Raikonnen and Romain Grosjean.
Button had initially complained of massive under-steer in his car, and struggled to get heat into the harder tyre.
“The car seems to be working pretty well on the softer of the two tyres, but on the hard tyre I just can’t get it working,” he said.
Ferrari endured a miserable second session, with home favourite Fernando Alonso a disappointing 14th, over 1.2 seconds off the pace, and nearly two tenths slower than team-mate Felipe Massa.
It had all been so different in FP1, with Alonso topping the timesheets ahead of Vettel.
Despite the setback, Alonso claimed he was not disappointed by the result, the first since major upgrades were attached to his car.
He said: ”Yesterday we said we try to recover some of the gap, we do the first step here in Barcelona. I’m confident that we did the job.
“The car, all the updates we introduced today, first numbers or first signs are positive. We need to look in more detail but everything looks as expected.”
Kamui Kobayashi continued Sauber’s excellent form with third in the first session, and completed a double top-ten finish with ninth in FP2. Team-mate Sergio Perez struggled in comparison, with a best finish of 12th.
Michael Schumacher, a six-time winner here, posted a best place of sixth.
Four teams used reserve drivers in FP1 today, with Williams’ Valtteri Bottas catching the eye with a stunning fifth quickest run, with first-choice driver Pastor Maldonado way down in 17th.
Jules Bianchi (Force India), Alexander Rossi (Caterham) and Dani Clos (HRT) came 18th, 21st and 24th respectively.
HRT’s problems continued in the second session, with Narain Karthikeyan unable to complete a timed lap due to electrical issues. Team-mate Pedro de la Rosa’s fastest lap was nearly five seconds away from Button’s time in FP2.
The Spanish team did not test at Mugello last week, and the team are at risk of missing the 107 per cent cut off point for a second time this season, meaning unless the other teams agree, they would not be allowed to race on Sunday.
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BIOGRAPHY: Mohamed Salah