United enjoyed victory at Crystal Palace on Saturday.
Vidic said: "It is difficult to get fourth because we are behind and we just have to focus on doing the best we can to get the fourth spot.
"It is not going to be an easy task, but we play against Liverpool at Old Trafford soon and I think that is going to be an important match for us.
"I believe that if we keep playing as we did against Crystal Palace - solid and improve our form again - we have a chance to do something good for the rest of the season.
"That means fourth place and Champions League. That is all that we are focused on and I think we will be all right now."
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