Louis van Gaal is eager for the Old Trafford club to push through a deal for the Arsenal center back, but the Catalans could still approach the Gunners this summer.
United is yet to agree to a deal to sign the center back from Arsenal, but Goal UK understands the Old Trafford club has maintained its interest despite having inquiries regarding a 7.5 milllion-euro bid knocked back.
Vermaelen is understood to be keen to leave Arsenal this summer, but Arsene Wenger wants to sign a replacement for the defender before the club sanctions his sale.
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And Barca could yet complicate the deal after rekindling its long-standing interest in the former Ajax man, who has 12 months remaining on his Arsenal contract.
The Catalan club recently completed the 20 million-euro signing of Jeremy Mathieu from Valencia, but is looking to bring in another defender this summer.
New United manager Louis van Gaal identified Vermaelen as a summer transfer target long before arriving at Old Trafford in July, and is eager for the club to push through a deal to sign the Belgium international.
Gunners boss Wenger has acknowledged that Vermaelen, who has found himself behind the first-choice center back pairing of Per Mertesacker and Laurent Koscielny at Arsenal, could leave this summer.
"I cannot guarantee [that he will stay],” Wenger told reporters during preseason. “But, should he go, I would bring another center back in for sure.”