This season the iconic Italian manufacturer have introduced several updates to their car.
However, the tweaks – and particularly upgrades related to the rear suspension – ended up derailing their campaign.
When asked where his team’s development had hit the buffers, he said: “It’s a big question.
“Obviously along the way something went wrong and we need to learn from it…
“We didn’t go and develop in the right direction, so clearly we missed something and not because something was on purpose by default set wrong.
“So it’s up to us to find a fix for it and make sure it doesn’t happen again.
“But it’s not so simple to find and fix it as it sounds.”
With the championship already won by Lewis Hamilton, Vettel insists Ferrari are almost back on track.
“We have got the car back to what we know and what we feel more comfortable with at the last two races,” he added.
“So hopefully we can keep that up and keep competitive here and in Abu Dhabi and give [Mercedes] a very hard time.
“Here it is always difficult to predict because in Interlagos there is something magical about this place and things happen. We will see how it turns out.”
Hamilton, meanwhile, says that with the constructors’ title in the balance he won’t be easing off.
“I’m in the frame of mind that I still have races to win and the job isn’t done yet,” said Hamilton.
“Right now, the focus is on making sure we secure this title in the next two races for the team.
“That’s the key, because, as we always talk about, there are over 1,000 people back at the factory that work so hard and that is the one that means most to them.
“That’s on my to-do list so we can collectively do that this weekend or next weekend.”
Hamilton has previously admitted he has no plans to gift any wins to Mercedes team-mate Valtteri Bottas.
He has won nine of the 19 races so far this season while Bottas, who has obeyed team orders to help Hamilton and endured tough luck while leading, is yet to secure a win.
Asked at the Brazilian Grand Prix whether he would let the Finn win were he in a position to do so, Hamilton told reporters that would be a decision for Mercedes to make.
He added: “It would be great for Valtteri to finish on a strong note, feel better in the car and finish off strong.”
“It’s not something I’ve asked Valtteri if he wants,” he added when asked if he’d reciprocate Bottas’ actions from Russia.
“But I don’t think he’s going to want that.
“He’s going to want to win it in his own way.
“So I don’t think I’ll be doing him any favours in the sense of giving him a win.
“I don’t think he needs that.
“I’ve not won here in Brazil for a long time, so I want to win.”