Balotelli has missed the last four matches and was given "a few days to think about his future" by Vieira when his teammates returned to training after the festive break.
The Italy international is expected to be absent when Nice face Bordeaux on Saturday, with reports claiming he is close to ending his stay at the club six months before the end of his deal.
When Balotelli arrived two weeks late for preseason training, Vieira asked Nice to allow the striker to leave but he has now left the door open for his former Inter Milan teammate to return to the squad.
"Mario is still here. He's a player who is under contract, but Mario is still here. Mario is still part of the squad," Vieira said.
"He's in a situation where he needs to think about his future in terms of what he wants to do and his options.
"We're waiting for him to come back to know what he wants to do. Nothing is impossible, from the moment when a player is under contract nothing is impossible."
Marseille, who had been expected to sign Balotelli last summer, are reported to lead the chase for the 28-year-old.