Button overcame balance problems to take his first pole of the season, the eighth of his career, and his first for McLaren.
"It's been quite a long time since I got my last pole position, back in 2009 in Monaco. It's been emotional," said Button, who won the F1 title that year with the Brawn team. "Maybe a five-week break between every race is what I need ... I'm getting old now."
The 32-year-old Button had understeering problems, but got it right when it mattered with a best lap of 1 minute, 47.573.
"A great qualifying session, the engineers have really been on it today," Button said. "I just got the balance right. We just tickled it, especially through qualifying. The balance is to my liking. It's difficult to find a car that works for me in qualifying but when it does we get pole position."
In one of the most unpredictable seasons for years, unheralded drivers Kamui Kobayashi of Japan and Pastor Maldonado of Venezuela finished second and third, respectively.
"In yesterday's practice we really struggled a lot, but this morning we improved a lot," said Kobayashi, who got his best ever qualifying position. "We didn't really know how much the track conditions would improve. Tomorrow is a different story, and we need to fight."
Maldonado, competing in only his second F1 season, has a good chance to secure his second victory of the campaign after winning the Spanish GP in May. He showed the potential of the Williams car, and was quickest in the first qualifying session.
"It will be important to get some points tomorrow, but if we can do more we will try," Maldonado said. "We were optimistic for this race, even though this morning we were a bit worried as the car didn't look that good. But I adapted to the different conditions."
Vettel's bid to catch championship leader Fernando Alonso looks to have taken another blow, while his Red Bull teammate Mark Webber will start the race with a five-place grid penalty.
"There was no problem, I just wasn't quick enough," Vettel said. "The speed wasn't there."
Alonso was sixth in qualifying behind Sergio Perez and Kimi Raikkonen.
Alonso posted the fastest time in Saturday's third practice, where drivers benefited from good conditions to fine-tune their cars for qualifying.
Heavy rain marred Friday's two practice runs, restricting drivers to a handful of laps, so the emphasis was on cramming as much testing as possible in sunny conditions at the Spa circuit.
Alonso, who is chasing his third Formula One title, had a best lap time of 1 minute, 48.542 seconds in his Ferrari. Kimi Raikkonen of Finland, the 2007 F1 champion, clocked 1:48.683.
Mercedes driver Nico Rosberg pulled out after five laps and later incurred a five-place grid penalty for a gearbox change.
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