It means Vettel could be champion next weekend in Japan if he wins and Alonso finishes eighth or lower.
The championship leader insisted, however, that he is simply focusing on making sure he wraps up the title eventually.
"I am not trying to think about it, to be honest," said Vettel. "I try to focus more on the present. I think we had an incredible chance two years ago to do so [at Suzuka], and we did it, but there are still a lot of points to get.
"Even though it looks very good for us there is still a chance for Fernando, so we have to stay on top of our game, but to be honest, we are just having a good time.
"We enjoy the fact the team is working really well, the car is working. It is on the edge, more so than you think from the outside, but it's nice when you get results like Singapore or this weekend
"I don't really care [when I win the title]. I look forward to Japan because it is one of the nicest tracks of the season."
Vettel said his victory, the third in Korea, had been tighter than it looked, and admitted he had been helped by the deployment of the safety car.
The Red Bull driver finished ahead of Lotus duo Kimi Raikkonen and Romain Grosjean.
"I think it was close. Lotus probably had more range today, I think we had more pace initially. I think Romain could have afforded to pit early and maybe we could not have reacted to that.
"I think it would have been close without the safety car, but in the end, Kimi was quite consistent, and we had two or three tenths on hand."
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