Fiorentina have collected a mere four points from the first 10 league games, the club's worst-ever start to a season, leading to protests from fans and the dismissal of Pioli.
The 53-year-old Vanoli, who was sacked by Torino in June, returns to Fiorentina, where he played from 2000 to 2002.
He will try to get Fiorentina off the bottom of the table on Sunday when they play at fellow strugglers Genoa.
