Alfa Romeo had already been the title sponsor of Sauber last year when the team was known as Alfa Romeo Sauber F1 team.
The 25-year-old became the first Latin American woman to drive a Formula One after she was appointed official test driver for the Ferrari-powered Sauber team.
It was Raikkonen'sa first win in more than five years and 113 races dating back to the 2013 Australian Grand Prix – and his first win for Ferrari since 2009.
Stewards ruled that Haas driver Romain Grosjean had caused a collision with Sauber's Charles Leclerc at the United States Grand Prix, and therefore received a grid penalty for the Mexican Grand Prix.
Switzerland's ATS news agency said the incident in May occurred when Raikkonen veered to avoid an oncoming vehicle and collided with a car parked to his right.
Despite having been replaced at Sauber for next season, Marcus Ericsson still hopes to take part in Formula One in 2019.
The 24-year-old had been competing for the role with Sauber's current driver Marcus Ericsson, who will stay with Sauber as their back-up driver.
20-year-old Leclerc - a Ferrari Driver Academy graduate - was the 2017 Formula 2 champion, and made his debut in F1 this season with the Sauber team.
Raikkonen left Formula One two years after winning the world title with Ferrari and spent two years competing in the World Rally Championship, before returning to F1 with Lotus in 2012.
Vettel lapped the Monza circuit near Milan with a best time of one minute 21.105 seconds.
As speculation surrounding the 2019 drivers market intensified following Mercedes' confirmation that Lewis Hamilton and Valtteri Bottas are being retained, Raikkonen's seat at Ferrari was a focus of attention.