The Serbian captain was sent off late on during United's 3-1 loss at Stamford Bridge, which has left them 14 points adrift of league leaders Arsenal in seventh as they bid to secure a Champions League spot, rather than retain their crown.
When asked if United were too far behind leaders Arsenal to challenge for the title, Vidic told Sky Sports News: "Yes, I think so now we are too far behind.
"We have to focus now to get in the top three, top four. We have to be positive. It's not going to be easy but we have to now fight for different things.
"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."
United host Sunderland in the second leg of their Capital One Cup semi-final on Wednesday, trailing 2-1 from the first leg.