Webber sets pace in Korea practice
Championship leader Mark Webber led two of his title rivals in the second free practice session for the South Korean GP at a slippery new Yeongam circuit.
The Australian spun his Red Bull off at turn 12 before returning to the track and immediately setting the fastest lap of the day, a minute 37.942 seconds, in the afternoon session.
Vettel was seventh, the position Webber occupied in the morning.
McLaren's Jenson Button, the reigning champion, mirrored his overall championship position by ending both sessions in fifth place.
The Briton's car had to be doused with fire extinguishers during the second session as the rear overheated but there was no significant damage and he was able to get back on track.
A decent turnout of fans, many of them schoolchildren getting their first taste of Formula One, watched as drivers struggled to stay on a dusty track that has yet to witness any actual racing.
The struggling Hispania team again suffered a setback, with Sakon Yamamoto hitting the wall and stalling.
Drivers sent up regular puffs of dirt as they carved through the final corner of an anti-clockwise circuit that had resembled a building site only days earlier.
Although grip levels have improved, conditions are treacherous with the pit lane giving particular cause for concern.
Mercedes chief strategist James Vowles told the BBC that they were treating the track like "a living entity".
"Every time the drivers go out the track will feel different. The pit lane is very dirty and we are a little bit concerned. In eight years of doing this, it is the worst I have seen," he added.