Vieira left New York City FC to move to the Allianz Riviera dugout and had hoped to hit the ground running when his players returned on Monday (02.07.18)
However, 27-year-old Balotelli and Plea, 25, were no-shows and the former Arsenal midfielder admits he will need to do some investigating to sort things out.
Vieira said: "We were supposed to have all the players. Unfortunately, those two weren't there. We're trying to get to the bottom of it."
Balotelli, who scored 18 league goals last season, has been linked with a switch to Marseille while West Ham United are said to be keen on Plea.
According to French publication L'Equipe's website, Plea's absence was due to a problem with airline tickets but the boss made it clear he has no time for players who want to be elsewhere.
Vieira added: "My wish is to keep the players who want to be part of the project."