Grosso was dismissed on Sunday after Fiorentina made a disastrous start to the campaign, leaving them bottom of the table with just one goal scored and nine conceded.
Vanoli returns just three months after leaving the club and has been handed the task of quickly changing their fortunes.
The coach took charge of Fiorentina last November after Stefano Pioli was dismissed following a 10-match winless run. Vanoli helped the club move away from relegation danger and finish 15th, while also guiding them to the Conference League quarter-finals.
Their European run ended against eventual winners Crystal Palace.
Fiorentina confirmed Vanoli's return on Monday, describing his decision to come back as an "enthusiastic acceptance" of the challenge.
The club will hope the coach can have an immediate impact when Fiorentina travel to promoted Venezia on Friday.
Venezia have also lost their opening three league matches, meaning both sides will be desperate for a first victory of the season.
Former Tottenham goalkeeper Guglielmo Vicario has thanked Roberto De Zerbi for supporting his move to Juventus, while admitting there were "dynamics" at Spurs that were "not aligned."
Federico Gatti scored in the 92nd minute to rescue a 1-1 draw for Juventus against AC Milan at the Allianz Stadium, extending their unbeaten Serie A record against the Rossoneri to seven matches.
Fabio Grosso has been sacked as Fiorentina head coach after just three Serie A matches, following a disastrous start that left the club without a single point.
AC Milan head coach Ruben Amorim believes the club has one of the strongest squads in the world after the club was able to bring in several big-name signings during the pre-season transfer window.
