Victory in the FA Cup final could at least provide something to savour but a poor defence of their Premier League title and exiting the UEFA Champions League have been a lowlights.
City are subsequently expected to make several changes to their squad in the summer and Nasri could be one of the men to leave, although Vieira hopes that is not the case.
Amid reported interest in the 25-year-old from Paris Saint-Germain, Vieira told Radio Monte Carlo: "Samir could have done better. I think, he, himself, is disappointed with the level he played at for several different reasons.
"But in the end, he remains an important player for us.
"I see him almost daily. I think he has learned a lot this year. He has faced competition and, when he was not good enough, he has been taken off.
"It has been an important season for him and I am sure he will come back even stronger.
"I hope he will stay. Time will tell. It's important to build a team on the long term. Samir is a very good player, a young player and I hope he will stay with us for a long time."
Vieira also confesses City's season as a whole has been a disappointment, although the prospect of facing Wigan Athletic in May's FA Cup final provides some incentive.
He said: "Has this season been a success? No. The aim this season was to retain the title and do better in the Champions League so, no, you cannot say our season was a success.
"We're in the cup final so that is a good way of saving the season. But we had a team you would expect to do better. We could have done better."