Vieira told The Sun: "Flamini is a big loss to Arsenal because he has been fantastic in the past two years.
"If they carry on losing players like that then it will be a worrying time for them. When young players like that are allowed to run through to the end of their contracts it means the offer from Arsenal wasn't the best.
"That's maybe why he decided to leave and you have to respect his decision. Arsenal have a really young team. I believe they should keep most of the players if they want to be better than last year.
"Flamini has left and I don't know what's going on with Hleb. When players see other players leave like that then of course they will ask what Arsenal want to do.
"If they want to compete with Chelsea, Manchester United or Liverpool they have to keep their best players and, if the best players leave, then people will question what Arsenal want to do in the future. I think all the younger players deserve a good contract and if Arsenal mean to keep them they should offer them a good contract.
"They have shown how good they are, they are committed to Arsenal and the club must show how important they are by offering them good contracts.
"I stayed loyal because Arsenal wanted to compete against the best teams in England and in Europe. My contract was good as well so I didn't have any reason - either financial or sporting - to leave and other players deserve that as well.
"I don't believe Wenger should buy big stars because I believe the Arsenal team at the moment is fantastic. But they need one or two key players with experience to help them through the big games."