Yes, good point but I don't think Vidal has any intention of leaving anytime soon (he said so himself) so the release clause is a bit of an irrelevant one in my opinion. I think that sometimes, integrating those massive release clauses into contracts can unsettle professional footballers, makes them feel trapped.
Vidal knows how important he is to the team (Marcello told me that), so there's no need to compound the situation with excessive contract stipulations. If the time comes and another club wants to bid, he'll either wish to stay or leave, I don't think Juventus is the kind of club that would punish him for wanting to move on.
This is all theoretical though.