The former Chelsea and England midfielder has still not featured for the MLS club through eight games of the campaign after suffering a calf strain during preseason.
Lampard, 37, has missed NYCFC’s first eight games of the 2016 campaign after being sidelined at the start of preseason and is expected to sit out their ninth this weekend when Vieira’s side host the Vancouver Whitecaps at Yankee Stadium.
The Designated Player also struggled with the same recurring injury over the second half of last term following his summer arrival, racking up just 10 appearances with three goals and an assist, and he is starting to become a popular suggestion for the league’s worst-ever signing.
He has been seen in light training at times over recent weeks, giving Vieira some hope that he could soon be available for selection, but the Frenchman believes it would be unfair at this stage to put a date on his potential return.
Asked about Lampard’s injury status at his post-match press conference following Wednesday night’s 1-1 home draw with the Montreal Impact, the Arsenal legend said: “Lampard, I don’t know because we have to look at his injuries and look at the training he’s doing to see how he improves day after day. He’s on his way back and this is really positive
“We don’t want to put any date because it would not be fair to him. He’s working hard, he’s working well. When he will be fit, he will be in the squad.”
After missing the playoffs in their inaugural campaign, NYCFC are currently enduring a difficult start to Vieira’s first season in charge, having taken just seven points from their first eight games to leave them sitting in eighth place in the Eastern Conference.
They have now gone winless since beating the Chicago Fire 4-3 on the opening day after conceding a last-gasp equaliser against Montreal, but the former World Cup winner insists that his side’s results are not a true reflection of their performances.
"We are in a period where everything is going against us," Vieira added at his press conference. "But we will stick together, we will fight together and we will turn it around. We showed that by the way we played tonight.
"I'm feeling sorry for the players because they worked so hard and worked so well. They deserve more than they are getting at the moment."