The 33-year-old has struggled to show his best form at Eastlands since arriving from Serie A.
The former Arsenal star enjoyed arguably his best game for the club in last weekend's win over Aston Villa and is determined to help City overcome Tottenham in their fourth-spot showdown on Wednesday.
"I have not given what the people who brought me to the club expected. But it is never too late and Spurs is another big game," said Vieira.
"My focus is how I can help the club get fourth place because I have been really frustrated.
"I don't feel I have been doing what I am capable of and I have not been playing at my best.
"It has been difficult because when I came back to England to join Manchester City, I was injured and it needed a strong belief from the board and from the manager. It is a good time for me to show they have made the right decision."