Goals from Gervinho, Lukas Podolski and an injury-time effort from Aaron Ramsey sealed the win for the Gunners, but they went in all-square at half-time following Konstantinos Mitroglou's equaliser in first-half stoppage time.
"Yeah I think in the first half [it was a struggle]," Vermaelen told Sky Sports. "The second half was better, but in the first half we were a little bit sloppy in the passing, we lost a lot of balls. We started well, but the first half was not great."
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He added: "We started well, a high tempo - that was great, but afterwards we dropped a little. I don't think it was because of the weekend but sometimes that can happen.
"[Bould] wasn't angry. It's not always good to be angry. He said things we did wrong, or that we could do better, and in the second half we were a lot better I think. 'Be more cautious on the ball, don't lose too many balls,' that's what [Bould] said."
Vermaelen also heralded the confidence that winning their first two Champions League games brings, and stated that he was happy for Gervinho, who has been in good form so far this season.
He continued: "It gives us a lot of confidence, getting six out of six [points]. That's really important.
"I think the most important thing is to win your home games in the Champions League, and we won an away game at Montpellier so we want to keep it going.
"[Gervinho] is doing great. He had a great pre-season as well. And now you can see in the matches, he's scoring a lot of goals, he's quick, he's quite busy, so I'm happy for him."