It was the Belgian defenders poor clearance from a Rafael cross which fell to the feet of former Gunner, Van Persie, who finished in a wonderful fashion. Arsenal went on to lose 2-1 at Old Trafford.
"The error of Vermaelen is unwelcome. The big defenders dont make this kind of errors," Wenger said. "When you give the ball to RVP in this situation, there are two chances on three, there is a goal."
Wenger was also unhappy with the decision in the second half to send off Jack Wilshere, who was booked for a second time after a late challenge on Patrice Evra.
"Yes [Wilshere's sending off] looked harsh from the stand, the second yellow," Wenger said.
United was close to going two goals up before halftime when a penalty was awarded for hand ball after the ball was adjudged to have hit Santi Carzorla's hand.
"I think we were a bit unlucky, especially with the penalty decision, we always have a penalty against when we play Manchester United, us so we get used to it," Wenger said.
"We had the better chances than but overall in a game like this you ought to not put yourself on the back foot when you are one down."When asked how the he felt when Van Persie scored the first goal, Wenger told Sky Sports: "I don't care who scores the goal, you want to win the games.
"That doesn't matter"