Vettel has endured a miserable season to date, claiming just two podiums to lie sixth in the drivers' championship, 122 points adrift of leader Nico Rosberg.
After earning a place on the fourth row of the grid at Monza, where the Mercedes-powered cars proved dominant as anticipated, Vettel expressed pessimism ahead of Sunday's race.
"Firstly, we always knew that this would again be a difficult weekend for us, so the situation doesn't come as a huge surprise," said the German, who at least managed to out-qualify team-mate Daniel Ricciardo, ninth on the grid after winning at the Belgian Grand Prix last time out.
"We have to take it as it comes," Vettel added. "Daniel showed in Spa that a lot is possible in the race.
"Of course I would have liked to have finished some spots closer to the front on the grid, so my expectations rest on the fact that our race speed is always a bit better than our qualifying speed - especially in comparison to McLaren. Ferrari is hard to judge and Mercedes and Williams are clearly too strong.
"The good news is that this is a track where you can overtake. But there is no point in getting over excited: I think that P5 would be a good result."