The Gunners will be 11 points behind Liverpool if they lose at the Emirates.
Vieira told The Sun: “Liverpool on Sunday will be a pressure game. It is a must-win game for their title hopes.
“Having beaten Manchester United and Chelsea you would expect them to be closer than they are in the table.
“But, instead, they have lost too many games against the smaller teams and have fallen behind.
“If they don’t win on Sunday then it will be very tough for them to come back.
“They have the character to do it. They showed it against Chelsea, coming back from 1-0 down. There are so many positives they can take from that Chelsea game that I’m sure they can do it against Liverpool.”
He added on manager Arsene Wenger: “My message is to be patient and calm. He knows what he is doing, of course.
“Arsene has been fantastic and committed to the club for the last 12 years.
“Questioning his commitment and quality as a manager is really difficult to understand.
“I know the Arsenal fans are frustrated because they want to win trophies. They know that the talent is there but they are missing something.
“Yes, maybe it’s that winning mentality — but it doesn’t come in one day.
“I don’t know if they need to get new players to be stronger. But they need to get a bit more togetherness, especially now because the title race is not over.
“I have not written them off at all. It is a period now that they have to come out together as a team. But I still believe they can beat anybody.
“It’s been a really difficult period on and off the pitch. So many things get into the team and affect the players.
“I don’t think the off-the-pitch things helped. What is important now is how they come together.”
