The club has offered no immediate explanation for Vidic's surprise absence and there was no hint of any issues when he attended new boss David Moyes' first press conference last Friday.
But Vidic's absence may be linked to the treatment he has been receiving to his troublesome knee injury that forced him to miss around 12 months of action.
Robin van Persie and David de Gea both miss Saturday's opener in Bangkok as they have been given extra time off, though they will join up with the squad in Sydney.
Chris Smalling (foot), Ashley Young (ankle) and Nani (broken nose) also miss out because of injury.
Shinji Kagawa will also be a late arrival - he has been given extra time off following the Confederations Cup and he will link up with the squad in Japan on July 21.
United's encounter with Singha All Stars in Bangkok on Saturday is a sell-out.
Defender Michael Keane, and midfielders Jesse Lingard and Adnan Januzaj have been drafted into the touring squad, with so many senior men absent.
New signing Wilfried Zaha also travels, and Moyes said: "He has only been with us for two days but I will take him away, see how he goes and have a look.
"This club has a great record of putting young players in the team. Hopefully Wilfried can be one of them."
Wayne Rooney will be with the squad with Moyes again insisting that he has no intention of selling the England striker.
"The club have already stated Wayne is not for sale," said Moyes. "I have met Wayne two, three, four times. A lot has been said about myself and Wayne over the years but we have a really good relationship. He came to my house, I have been to his recently to have a chat. Also he is here every day.
"I will say he is training great. They are telling me it is the best shape they have seen him in for a long time.
"From that point of view I am taking it as a positive and I am going to try and get the best Wayne Rooney I possibly can."