The German finished first in Japan on Sunday and now has a 90-point lead over the Ferrari driver with just four races remaining.
The season would need to take a bizarre turn of events to deny Vettel his fourth consecutive title. Alonso would have to win three of the remaining four races and finish second in the other, with the Red Bull driver retiring from them all.
"Even if Vettel doesn't finish all of the races, I need to win nearly all, so it's a matter of time [until Vettel is champion]," Alonso told reporters.
Alonso finished fourth in Japan.