Grand Prix at Interlagos with a healthy 13-point lead over Ferrari's double champion Alonso.
Vettel goes into his 101st race buoyed by a fine run of recent form which has seen him reel off four straight wins before finishing third in Abu Dhabi, after starting in the pit lane, and then second in Austin last Sunday.
On Sunday, all the German has to do is finish in the top four to take his third successive world title.
"It is sport and anything can happen," said Vettel, who won at this track in 2010.
Alonso, in dramatic contrast, has yet to taste victory at Interlagos in nine previous visits, finishing second on two occasions.
Vettel added: "We have to go step by step, to make sure we get the maximum result. Historically we have been very quick here. We know it is a place where lots of things can happen. We need to be sharp in the moment to see what we can get."
Vettel struck the first blow in Saturday's qualifying when he secured fourth place on the grid with Alonso down in eighth before the Spaniard was bumped up a spot when Williams driver Pastor Maldonado was handed a 10-place penalty.
Asked if the championship situation had affected his performance, Vettel replied: "Not too much. I've just come out of qualifying knowing I could have done a little bit better.
"The first run in Q3 was not good for me as I did a mistake going into Turn Four. The second run I was pretty happy with it but it wasn't quick enough."
He added: "Obviously (teammate) Mark (Webber, who placed third) was a bit quicker, but I think we are in good shape and we've been competitive all weekend."
Alonso admitted he needs a "crazy race" if he is to beat the odds and land his third drivers world championship.
"We need some strange circumstances, we need a chaotic race in a way, rain or something, let's hope we see something like that," he said.
Alonso declared himself content with his qualifying laps.
"It's not a surprise because we started seventh in Abu Dhabi, ninth in the USA and seventh here. So it's more or less as expected," he said.
Alonso denied he and Ferrari had gone for a fully wet set-up in qualifying in anticipation of Sunday rain.
"We ran with maximum downforce in the dry so obviously it is the same in the wet, so no big changes," Alonso insisted.
"I don't think anyone gambled today for a wet set-up because there is minimal changes."
Lewis Hamilton will start his 110th and final race for McLaren from pole position with Jenson Button completing their fourth front row lockout of the season.
In his final qualifying session for McLaren before leaving to join Mercedes next year, 27-year-old Hamilton clocked a fastest lap of one minute and 12.458 seconds to outpace fellow-Briton Button by one-tenth of a second.
"What a great feeling it is. What a great weekend so far and I hope we can push on Sunday. It is great to be one-two with Jenson Button. I am expecting a difficult race," said Hamilton.
Sunday will also be an emotional day for seven-time world champion Michael Schumacher who will head into retirement having qualified in 14th place on Saturday.
The 43-year-old German is not going quietly, however, claiming his Mercedes team compromised his car set-up too much in qualifying in the hope that it rains on Sunday.
"We might have compromised the car too much for the rain which is expected for tomorrow and was also forecast for qualifying," he said.
"When the drops came down just before, it looked even more likely. Anyway, my starting position is now quite tricky, and I will have to see what we can make out of it.
"Of course, coming from where I am does not make it easy, but I will definitely try to fight my way up the field."