By Alan Baldwin

SAO PAULO | Sun Nov 25, 2012 5:25pm EST

(Reuters) - Red Bull's Sebastian Vettel became Formula One's youngest triple world champion at the age of 25 after a wet and chaotic Brazilian Grand Prix rollercoaster won by McLaren's Jenson Button behind the safety car on Sunday.

The German, needing only a fourth place to join the greats as the first driver to win his first three titles consecutively, finished sixth after fighting back with a damaged car from last on the opening lap.

It proved enough after his sole rival Fernando Alonso, needing victory, crossed the line in his Ferrari in second place with Brazilian Felipe Massa third.

"You're a triple world champion. You're the man. You are a triple world champion," shouted team principal Christian Horner over the team radio as Vettel crossed the finish line after what he said later was the toughest race of his career.

The team could not hear his reply, or his choked sobs of joy, because the radio was faulty and had been only incoming for most of an afternoon that threw one obstacle after another at the champion.

Vettel ended the season with 281 points to 278 for Alonso, who would have been the youngest triple champion at 31 had results fallen his way. Kimi Raikkonen was third overall for Lotus on 207 in his comeback year.

Red Bull had already won the constructors' title for the third year in a row.

"It's very difficult to find the right words, especially after the race today," said Vettel. "I think everything that could go wrong, went wrong.

"To limp home, underneath the safety car (conditions), I didn't know if it was enough...we had to really fight until the end."

On a day when the Brazilian weather produced a thriller to stand the test of time against some of the sport's great races, it seemed the gods were on Alonso's side almost as soon as the starting lights went out.

Vettel was squeezed from fourth place, fell back into the pack and was caught in a collision with Brazilian Bruno Senna's Williams that spun him around helplessly facing his speeding rivals.

"There is visible damage, it is not the front wing, we cannot fix it," he was told as he rejoined the chase with the whole race ahead of him.

Four laps later Vettel was assured the data looked good but a concerned-looking Red Bull technical head Adrian Newey had a photograph of the damage taken at the driver's pitstop to get a closer look from the pit wall.

SAFETY CARS

Alonso, Vettel's only title rival who had 13 points to make up, looked like he could steal it as the championship pendulum swung both ways over the 71 laps at Interlagos.

The safety car was deployed twice, there were crashes, collisions, botched pitstops and constant uncertainty about the weather with black clouds overhead, occasional rain but not the torrential downpour many had feared.

"We lost the championship before today, not in Brazil," said Alonso, who agreed with others that it was one of the hardest races he had ever driven.

"It was a lot of risk every lap to crash and have an accident. We could not afford that for sure because we needed a podium finish to have any chance so it was a very delicate situation."

Button, who also won the first race of the season in Australia, took his career tally to 15 wins.

McLaren's Lewis Hamilton had led from pole position but could not shake off his team mate, who made a wise decision to do a long first stint on slicks along with Force India's Nico Hulkenberg.

Hamilton's last race for McLaren before he joins Mercedes ended when he and Hulkenberg collided on the 54th lap as they came up to lap back markers into turn one.

Hulkenberg, who had led for much of the race and skidded into the blameless Hamilton, was able to rejoin but was given a drive through penalty that ended his hopes of a podium finish and seemed harsh given the treacherous conditions.

"I am happy for the team that we got a win and feeling good for the future. I feel numb, a little bit like I did in 2007, it is mixed emotions at the moment," Hamilton said, referring to the title he missed in his first season by a single point when he finished seventh in Brazil.

Australian Mark Webber finished fourth, ahead of Hulkenberg in fifth in his last race for Force India before moving to Sauber.

Seven-times world champion Michael Schumacher took seventh place for Mercedes in his last race in Formula One, moving over to allow his friend and compatriot Vettel to take sixth and join him as one of only three drivers to win three titles in a row.

The two Germans embraced each other at the finish, with Red Bull staff swiftly donning 'V3ttel' T-shirts as the champagne celebrations kicked off with Vettel also making a point of hugging Alonso and Ferrari team boss Stefano Domenicali.

"I'm finishing off and he's clinching his third title. I'm very proud of him and he's a good friend of mine and lets see what happens in his future," said Schumacher.

Out of the points, Caterham celebrated 11th place for Russian Vitaly Petrov that lifted them back ahead of Marussia into 10th place overall in the championship - a finish worth millions to the team in prize money.

"Disappointed doesn't even come close," said Marussia principal John Booth.

(Editing by Ed Osmond)