Vidic has been out of action since the warm-up for the FA Cup defeat against Leeds United on January 3.
Mladenovic said: “The World Cup is drawing nearer and we want to be kept up to date with our players’ fitness, but in Vidic’s case, we simply don’t know what is going on.
“It’s been 10 days that I’ve been waiting for a response from Manchester United on the nature and severity of Vidic’s injury, but it still hasn’t come.
“We had expected to see United’s physician [Steve McNally] at Uefa’s medical seminar in Stockholm, but he wasn’t there. We then sent a written request to be provided with Nemanja’s diagnosis and any treatment documents, but we have had no reply so far.”
Serbia coach Raddy Antic said: “It is only a matter of time before he will be in top shape. He told me that he wants to help the club, but that he does not want to make his injury worse.”