The 21-year-old struck three times in the first half with Valentin Eysseric adding a fourth after Artur Ionita got one back for Benevento before the hour mark.
Fiorentina's first win in four games pulled it away from the relegation zone into 12th position with Benevento, which had been equal on points with the Tuscany side before the match, dropping to 16th.
Vlahovic opened the scoring after eight minutes taking advantage of an acceleration from Franck Ribery and an Eysseric cross to finish off.
The French pair again combined to set up the second after 26 minutes with the Serb picking up a Bartlomiej Dragowski cross to curl in the third from a distance before the break, to bring his tally to 12 goals this campaign, twice his tally from last season.
Benevento reacted well with Ionita reducing the deficit 10 minutes after the break but Ribery helped snuff out a revival setting up Eysseric to send in the fourth.
Fiorentina next week hosts second-placed AC Milan before a trip to lowly Genoa, which earned a point in a 1-1 draw at home against mid-table Udinese.
Eighth-placed Sassuolo kept its UEFA Europa League hopes alive with a 3-2 win over European rival Hellas Verona, just behind in ninth.
Serie A holder Juventus heads for Cagliari on Monday (AEDT) after its UEFA Champions League elimination, looking to close the gap on leader Inter Milan, which travel to lowly Torino.