"There was not just one weak point," Vasseur told France's Auto Hebdo. "Instead, it was the result of the difficulties of the team in 2016."
Indeed, Sauber almost collapsed financially in 2016, and was ultimately rescued only in the late summer by a consortium linked to driver Marcus Ericsson.
Vasseur explained: "The 2017 car was ready very late. It was a new project and if you are six months late, it only adds to a difficult situation and it's very difficult to catch up.
"The decision to use the 2016 Ferrari engine last year also did not help," he added.
"Now we have a brand new project. Sauber started very late last year, but the 2018 project is on schedule," said Vasseur. "But of course only the first test in Barcelona at the end of February will tell us where we are."
Sauber has ousted Pascal Wehrlein for 2018, but replaced him with the highly rated Ferrari junior Charles Leclerc, who heads into the season as the reigning F2 champion.
"Yes, there are many expectations and I think we have made some strong choices," Vasseur said.
"Above all with the engine -- having the 2018 Ferrari engine will mean a big step forward for us. And we have started with a new philosophy for the car."