"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."
"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.
"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."
"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."
