Two ways.
A) Off the two clubs, Juve have players that Fiorentina can potentially employ on their pay roll and use to substantially strengthen the team. The potential makeweights with the right financial offer.
B) Player will. JoJo is already in Italy, and moving to one of the Italian big clubs maybe a kore tempting move for him without being completely unsettled, add to the fact that Vucinic would be a much bigger comforting factor in settling in and playing for the team.
If we are serious enough, JoJo will change his mind. Very hard to say no when an undefeated, title winning Juve come calling offering you a chance to make a name for yourself on the biggest stages. Something Fiorentina, unfortunately and with due respect cannot match. That said, if we don't get him this year, I see him being bought by PsG next year, unless Fiorentina win the league.