Vettel locked his tires on the first turn as the Ferrari of Felipe Massa pressured him from behind, but after that the German consolidated his advantage to beat Alonso and Red Bull teammate Mark Webber, who finished third.
With seven races remaining, it looks increasingly likely that Vettel will clinch a fourth straight Formula One title — unless he has an alarming drop in form or Ferrari finds a way to get faster.
It was Vettel’s third career win at Monza, his sixth of the season and 32nd overall — moving him level with Alonso’s total wins.
He was more emotional than usual — pumping his fist several times as he walked up the steps to the podium — and with good reason. He now leads two-time former champion Alonso by 53 points with 175 available.
The last driver to win here from anywhere other than pole was Rubens Barrichello from fifth spot on the grid in 2009, and Alonso never really looked like doing that after starting from the same position.
Vettel shook his head vigorously in celebration and let out his trademark high-pitched whoop as he celebrated his third win in the past four races. Given his tough start, it was arguably even more impressive than his dominant performance at the Belgian GP two weeks ago.
“Very good boys, very good race,” Vettel yelled over his car radio when he crossed the line. “Sorry for the lock up into the first corner, made the first stint a bit tricky.”
Massa finished fourth ahead of German drivers Nico Hulkenberg — a season’s best for Sauber — and Nico Rosberg, on a poor day for Mercedes.
There was light rainfall about one hour before the race, but the sunshine returned for the start as the top 10 on the grid opted for medium tires.
Vettel started from pole for the fourth time this campaign and 40th in his career, but the first since the Canadian GP in June.
He struggled to hold on to the lead after the first turn, with Massa right on his back. The Brazilian had boldly passed Hulkenberg and Webber to jump from fourth to second.
Roared on by thousands of flag-waving tifosi, Ferrari had a point to prove and Alonso brilliantly overtook Webber on the outside to move up to third and then zoomed ahead of Massa.
After 10 of 53 laps, Vettel led Alonso by about five seconds, but engineers told him that his front right tire had taken some damage on the first turn and Alonso began to shave some time off.
Vettel pitted on lap 24, switching to hard tires, followed shortly by Webber and Massa, but Alonso stayed out on his tires until lap 28 as leading drivers chose a one-stop strategy.
In his last ever European race before he retires from F1 at the end of the season, Webber got his first ever podium at Monza. Vettel won his first race here for Toro Rosso in 2008 and repeated that with Red Bull two years ago.
Briton Paul di Resta’s race lasted only a few seconds after he sustained damage to the front left tire of his Force India car. He appeared to hit the back of Romain Grosjean’s Lotus and stewards were investigating the incident.
Di Resta has had some bad luck recently, failing to finish the Belgian GP after qualifying in fifth because Pastor Maldonado crashed into him.
Kimi Raikkonen also struggled and had to pit early with front-wing damage to his Lotus.
Lewis Hamilton, starting from 12th after his worst qualifying performance since the Spanish GP in May of last year, had a frustrating afternoon. First his team told him they could not hear him over race radio, then he had to pit to repair a puncture. But he made up ground to finish ninth.
Frenchman Jean-Eric Vergne pulled over to the side of the track on lap 16 and abandoned.Felipe Massa, Fernando Alonso, Formula 1, Formula One, Italian GP, Italian Grand Prix, Mark Webber, Monza, Sebastian Vettel