The Daily Star says opening negotiations have begun towards a deal that would see Vidal become the most expensive signing in United's history.
He has just returned from a break following his World Cup exertions in Brazil, but has told friends he is keen on joining United.
Juventus chiefs are willing to do business if United match their asking price, but the move appears to hinge on Vidal proving there are no genuine problems with his long-term fitness.
A source said: "No fee has been agreed yet, but United are increasingly optimistic they will be allowed to put Vidal through a medical before any price is discussed properly.
"They basically want to check on the knee and make sure he is fit before going through the process of talking money with Juventus.
"And it seems Juventus are happy to go along with it, even though it's not the usual way of doing things in a transfer."
For the big stories and the banter follow us on Twitter: @tribalfootball