Not sure what you mean by the last part really mate. Asking him to play as a holding midfielder would be restricting him to a role - the holding midfielder role. All that has been said is that a holding midfielder role doesn't get the best out of him. If you disagree that's fine but frankly I've not got the slightest doubt that he is best as a box to box #8.
And as has been said numerous times being able to play at right back, centre back or sweeper is not too relevant when talking about his best position. There isn't a person in the thread who is trying to portray Vidal as a limited player by saying he's better as a #8 than as a sweeper or right back. It's possible to recognise Vidal's versatility whilst being aware that he has a best position. In fact if you re-read my post I actually explicitly state that he could shield the back four, it's just a waste of his ability.
I'm aware that Pirlo plays for Juventus mate, the point stands that Vidal does not play as a holding midfielder. Whether it's Pirlo or Marchisio he plays ahead of both contributing offensively as a proper box to box midfielder. It's nothing like the role that is being discussed in this thread. Your comment that it's the same because both teams play 3-5-2 does not have much weight IMO considering the seperate versions of the system that are being used - which are significantly different - and Vidal's different role in both of those teams.
It's just a less extreme version of the Chile example, suggesting its the same because Chile and United play a 3-5-2 when for Chile Vidal played as a #10 ahead of two midfielders but here he would be playing as the deepest midfielder in the team.
Like I said most of the concerns that have been raised have been valid but perhaps slightly overplayed. I agree with the view that telling him to sit deep would be a waste - not because he can't do it before you respond - but because Vidal has much more to offer than that. If you sign the best midfielder in the world for £40-50million then we should be playing him in his best role IMO.
But I don't think it even needs to come to that because I think playing him next to Herrera as two box to box midfielders would be fine in a 3-5-2. There is enough defensive protection there to make that work and I don't see the requirement for a holder in those circumstances, particulalry if we push our line up 10 or so yards.