Vidic is confident that United will once again find the strength from within to succeed.
The United captain said: "I see headlines that a player has signed for City, then a second, then a third. It is clear they have the money and they will sign new players. But we have to focus on ourselves. We have to think about what happens behind our doors not our neighbours.
"They will definitely be in position to fight for the league but I think we have enough good players and a good team to challenge for the trophy."