The French defender received a large amount of criticism for his displays during his first season in England.
However, Vermaelen has revealed that he never doubted the former Lorient player's ability.
"He has done amazing this season," the Belgian told the club's official website.
"I don't understand why a lot of criticism was there in the first season. The first week I saw him you could tell he was a great player and now it is all coming out in his second season. I am really happy for him."
Koscielny has featured more this season than any other Arsenal defender.