It was the final action of the season for the Impact, who played a pair of friendlies in Italy after the end of the Major League Soccer regular season. They lost 1-0 to Di Vaio's former club Bologna last week.
Di Vaio took a pass from midfielder Andrea Pisanu on an indirect free kick and curled it into the top corner from 10 yards in the 25th minute. Pisanu, a Bologna player who was on loan to Prato of the second division last season, is on a try-out with Montreal.
"Great win for the team and what an experience it was to play against Fiorentina," midfielder Calum Mallace wrote on Twitter. "Will never forget that one."
Fiorentina stars Luca Toni, Alberto Aquilani, Pizarro, Manuel Pasqual and Borja Valero saw playing time, mainly as substitutes.
Interim coach Mauro Biello, filling in for the fired Jesse Marsch, singled out the defence and goalkeeper Evan Bush for praise as Montreal overcame some early troubles dealing with the skill and team play of the Italian side and held on in the second half for the win.
"It was great to get a chance to play a first division team, with the quality of players they had," Biello said on a conference call from Florence, Italy. "Our players reacted well.
"We tried to play a compact game and play the ball up the wings and use our speed."
Bush dove to his right to stop a long distance drive by Fiorentina midfielder Alberto Aquilani in the 11th minute and pushed away a shot off a give-and go from Adem Ljajic. Two minutes later, he made another save when Cristian Llama fired from near the left corner of the box that dipped suddenly.
Among other trialists, Oussama Essabr, played the first 45 minutes and Nicolas Cordova came on in the second half.
Biello said it would be up to management whether any of them play for Montreal next season.
He said he was grateful for the chance to show what he could do as a coach during the two-week trip to Italy, he said the club is still searching for its next head coach.
Midfielder Patrice Bernier acted as captain.
The Impact will now split up for the off-season and regroup Jan. 19 in Montreal for physicals before the start of training camp two days later.
"Great win against Fiorentina, and now it's vacation time!" defender Hassoun Camara tweeted.