It depends on Vidal.
In Real he'll get more money, fame and much bigger chance to win the CL trophy. They are a much more attractive club than Juve, especially to someone who comes from a Spanish speaking country. Even Italians find it more attractive than Italian top clubs (Pirlo said it in his biography).
Vidal's agent will push for it, while Real will find a way, if they haven't already, to speak to Vidal and try to convince him to join. I think those guys can be really persuasive.
Then we will be faced with an option to keep him, risking to have an unhappy star in our team, or sell him for as much as we can get. We won't get a Bale amount, that's certain. Real won la decima, and Vidal isn't worth to them too much. They already showed that Xabi Alonso and Modric in midfield can take them far. This excludes a crazy amount. To us he's worth more than 50m eur, but this might actually be the medium ground for both teams.