Alex Ferguson's side have conceded nine goals in their opening
six league matches - their worst record at this stage since
2001-02.
And their backline has been particularly leaky on the road with
seven goals against them in just three games, including two at
Bolton on Sunday.
That could spell trouble for United in Valencia in their crucial
Champions League clash ahead of Saturday's trip to Sunderland.
And skipper Vidic warns they will have to improve at the
Mestalla as Unai Emery's men are in fine form.
Despite selling David Villa and David Silva, Valencia currently
top La Liga and also thumped Turkish outfit Bursaspor 4-0 in their
opening match in Europe.
Vidic said: "We have to be much better in Spain than we were at
Bolton.
"Valencia are in decent form and won their first match 4-0.
"They are also top of La Liga and that shows how well they are
playing - whereas the last few games we have played away have been
very disappointing.
"It is a big concern conceding so many away goals. We know over
the last few years we haven't given many away.
"We have always looked strong and haven't given teams so many
chances.
"But in the last three away games we haven't kept a clean sheet.
We have lost too many goals and have to stop that.
"This is not the true Manchester United. It does have to be
sorted quickly.
"In the last three games we have conceded the type of goals from
set-pieces that we haven't conceded in the last four years."
Vidic is set to resume his partnership with Rio Ferdinand in
Spain. The England skipper was left out at Bolton because Fergie
does not feel he is ready for more than a game a week following his
knee injury.
But Vidic hopes United can get back to the sort of solid
defensive performances which were their trademark.
He said: "Our defending has always been our platform. We need to
work on the small details."
