Born in Senegal, Vieira's family moved to Paris when he was aged 8; he did not return to Senegal until 2003. Vieira first played for AS Cannes, where he made his debut at the age of 17 and captained the team aged only 19. Soon after that, in the summer of 1995, he was snapped up by Italian giants AC Milan, though he mainly stayed in the reserves and only made two appearances for the first team.