Reigning world champion Vettel was quickest yesterday at the end of both 90-minute sessions at the Hungaroring in his Red Bull.
There will be no qualms within the Red Bull garage, however, as the team opted to run the rule over their long-run pace.
Vettel cranked out the laps, completing a high of 27 overall in the hour-long run with temperatures soaring, hitting 32 degrees and 47 on track.
The current championship leader managed a quick lap towards the end, finishing fourth overall behind Grosjean in his Lotus.
The Frenchman lowered Vettel's best from yesterday by half a second, setting a time of one minute 20.730secs.
Ferrari's Fernando Alonso, Vettel's nearest title challenger but 34 points adrift, finished 0.168secs down.
McLaren continue to show considerable improvement, with Sergio Perez third quickest and 0.322secs adrift, with Vettel in close attendance.
Perez, however, blotted his copybook late on as he lost control of his car before sliding sideways into a barrier, ensuring his team face a tough lunch break to prepare it for qualifying.
Ferrari's Felipe Massa was fifth on the timesheet, just over four tenths of a second down, followed closely by Lewis Hamilton in his Mercedes.
Mercedes have claimed six of the last seven poles, but the change of Pirelli's tyres for this event so far appears to have affected them.
Following a series of blowouts at last month's British Grand Prix, Pirelli have been forced to alter their tyres for safety reasons, marrying last year's structure with this year's compounds.
Mercedes also missed out on valuable running on the new rubber at last week's young driver test as they served a ban for conducting their own private test with Pirelli in May.
Hamilton's team-mate Nico Rosberg was down in eighth place, six tenths off the pace, just behind Vettel's colleague Mark Webber.
McLaren's Jenson Button and Adrian Sutil in his Force India rounded out the top 10, with Lotus' Kimi Raikkonen 11th, almost nine tenths of a second behind team-mate Grosjean.
Paul Di Resta was 13th in his Force India, 1.2secs down, with Max Chilton 21st for Marussia, albeit as Esteban Gutierrez failed to set a time due to technical problems with his Sauber.
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