Premier League - Arsenal beat United to reignite title battle
Aaron Ramsey scored a deserved winner on 56 minutes as the Gunners emerged from a torrid run of form with a vibrant display.
Although Arsenal kept their slim title hopes alive, the main beneficiaries were Chelsea, who are three points behind leaders United with three matches left - the top two meet at Old Trafford next Sunday.
Having seen their trophy hopes disintegrate over two miserable months, Arsenal entered May freed from the pressure of expectation and produced one of their best displays of the season despite missing their captain.
Despite Arsene Wenger's side's spring meltdown, their commitment to attacking football remains undimmed.
And the winger was denied a certain goal when Patrice Evra make a crucial sliding interception on 15 minutes.
Just after the half-hour came a moment of supreme cynicism from Vidic, as Walcott bent a cross towards the head of Robin van Persie.
Knowing he could not reach the ball by fair means, the United captain flicked out a hand and batted it to safety.
The offence was missed not only by referee Chris Foy and his assistant, but also by most of the Arsenal team, of whom only Van Persie protested with any vigour.
United may point out the questionable decisions that gave Chelsea victory against Tottenham on Saturday, but two (or even three) wrongs seldom make a right, and a damaging weekend for the refereeing profession got worse.
The visitors' best chance came from Fabio, who latched on to a superb Nani pass but dithered when faced with the option whether to pass or shoot.
Wayne Rooney was a peripheral figure and picked up a silly booking when he kicked the ball at a prone Wilshere (not very hard, admittedly) after pushing the Arsenal midfielder.
A half-time blast from Sir Alex Ferguson inspired a much brighter United performance after the break, and Rooney brought a decent save from Wojciech Szczesny with a free-kick on 49 minutes.
But their resurgence was cut short by a man who rejected a move to Old Trafford before moving to Arsenal from Cardiff.
Ramsey produced a cool finish from 15 yards to place his shot low into the left corner following good work by Van Persie, giving Arsenal a deserved lead.
United surged back, and Vidic might have done better with his header after meeting Nani's cross.
Then Evra had a penalty appeal turned down after a coming-together with Clichy.
Szczesny excelled himself on 77 minutes when Rooney slipped Nani in down the left channel, but the goalkeeper stood tall to repel the shot.
Arsenal have made a habit of conceding heartbreaking late goals, but this time they would not be denied. United remain favourites to win a record 19th league title, but their job has just got much harder.