The Red Bull driver is four points behind the Ferrari going into Sunday’s race, after which there are just four grands prix left in what has been a highly unpredictable season.
Vettel, the man in form after imperious back-to-back victories in Singapore and then Japan last week, clocked a time of one minute 38.832 seconds in the second session, 0.032 seconds ahead of fellow Red Bull driver Mark Webber.
The Spaniard Alonso, desperate to get back among the points after retiring in Suzuka, registered the third-fastest time, 0.328 seconds slower than his title rival Vettel.
The McLaren of Jenson Button was fourth fastest, next up was the Mercedes of Michael Schumacher, Felipe Massa in a Ferrari and Schumacher's team-mate Nico Rosberg.
Briton Lewis Hamilton, quickest in the first session at a cool but sunny Yeongam, could only clock the eighth-fastest time in the afternoon.