Vieira: United get home help from referees
"When United play at home they get some advantage that other teams don't get," the former Arsenal midfielder told BBC Sport at the Soccerex conference in Manchester on Wednesday.
"I think when you go to United, Madrid, Barcelona, or Milan, when the referees referee these kind of games, it's always difficult to go against these kind of teams. This is the way it is."
Frenchman Vieira, who enjoyed many title battles against United while at Arsenal, was speaking two days after Fulham had a strong penalty claim turned down near the end of their 1-0 defeat by the champions at Old Trafford.
Victory allowed United to go three points clear of City, with whom Vieira had a short spell as a player and is now a football development executive.
"It's something the teams who are used to winning get all the time, so we need to win games so we have this advantage in the future," he said.
Vieira, who made 28 appearances for City after joining from Inter Milan, said that despite his club seeing their lead disappear in recent weeks, they deserve to win their first English title since 1968.
"Since the start of the season we've been the best team and played the best football," Vieira said.
"I believe the club deserves it. When you are first you have the advantage, [United] are favourites."
Vieira also said the return of Carlos Tevez, out since September after refusing to warm up as substitute in the Champions League against Bayern Munich, was a boost.
"We're glad Carlos is back. The quality of these kind of players will be really important at this stage of the season," he said. "The issue was between him and the club. The players were really pleased to see him back."