My take on this is that something happened and Marotta took care of it, not letting media know, not to decrease Vidal's price even more.
Maybe they know his knee is more fucked up than we know;
Maybe he's been shitting all around Turin's nightlife more than we know, and hasn't been focused/applied on training;
Maybe they offered him a new contract and he refused, asking for too high of wages.
Only something like this could force Marotta's hand to the point of selling him for peanuts. Maybe Marotta was cautiously shopping him around, and the incident at Chile made potential buyers like Madrid / United go away.
Maybe the only other offer received was from Arsenal, refused by Vidal. And then there is Bayern.
If Marotta is suddenly selling him for 35M, it indicates a situation with little leverage we don't really know. And probably won't be publicly acknowledged by Marotta.