The 32-year-old, who was sent off in his side's 3-1 defeat to Chelsea on Sunday, also refused to blame David Moyes for the club's inconsistent form this season
The loss at Stamford Bridge was the club’s seventh in the Premier League this season, leaving the champions 14 points behind leaders Arsenal in the table.
"Yes, I think so now we are too far behind," Vidic told Sky Sports. "We have to focus now to get in the top three, top four. We have to be positive."
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Vidic, who was sent off in the second half against Chelsea, admits securing a top-four place is a difficult challenge this term but has urged his team-mates to find cohesion.
"It's not going to be easy but we have to now fight for different things," Vidic continued.
"We have to focus on the [Capital One] Cup and the Champions League.
"It's never easy but we have to stay positive, we have to stick together, we have to work hard to change the things around.
"Hopefully we can play better football and with a bit of luck I think we can achieve that."
Vidic also claims the United players are "tired" of questions regarding David Moyes and refused to blame the Scot for the club's form this season.
"Let’s focus on other games that are coming. We cannot always speak about managers - 'is he coming?' - you just keep coming back to these kind of questions," said the 32-year-old.
“I have to say the players are tired to speak about that any more, I think the manager’s trying to do the best he can."