The Gunners have suffered defeats to Norwich City, Schalke 04 and Manchester United in recent weeks, and Wenger believes that Vermaelen has taken the defeats personally.
Wenger said: "Physically, Thomas is outstanding, but what has happened to him in recent weeks has affected his confidence. You put more pressure on yourself when you are the captain. You want to be the best player in the team.
"However, you cannot be a defender and not make a mistake - but if you make one, then maybe you put a bit more pressure on yourself then because you think 'I am the captain and I cannot afford that'.
"Thomas is a guy who is completely involved in the game - he breathes his football from morning until night, he thinks about it [all the time] and wants to do well."
Arsenal host Fulham on Saturday.