Once the green flag was out, Da Costa was fighting to get past Dennis – a move round the outside resulted in contact, leaving the former pointing towards the barriers after a slide.
The Andretti driver was given a 5sec time penalty, taking the pressure off Mortara. Vandoorne was within range of Dennis and therefore was promoted from second to third, easily securing the title.
In what was the first championship for the Belgian since his 2015 GP2 triumph, and the Mercedes man implied he had learnt from his performances last year, utilising the experience to win this season’s title.
“World Champions, wow, it’s just the best feeling ever,” Vandoorne said after the race.
“Just look at the season we’ve had: the consistency, the car’s been amazing, the team has done an amazing job and everything single one of us deserves this. What we’ve accomplished is something special.
“It was really about consistency in the end, they [title rivals Evans, Mortara and Vergne] all had more wins than me. I’m drained after this year. It’s been a lot of effort that I put in, that the team put in. It’s incredible.”
Vandoorne and his team claimed both titles in what was its final race as Mercedes, with McLaren taking over the reigns next season.
The Belgian is thought to be headed to DS Penske next season, whilst his team-mate Nyck De Vries is expected to be confirmed at Venturi, which is set to become the Maserati works team.