The duo again kept a cleansheet in victory at Fulham on Saturday.
"They've done well, I must say," Wenger told arsenal.com. "Vermalen had a very good game against Norwich but we were on a run of good results and good stability and we don't concede goals at the moment so that's why I keep them [Mertesacker and Koscielny] together.
"It's a very difficult decision. Thomas is one of the best defenders in England, like the two others. I told them at the start of the season we have three outstanding central defenders.
"It's about game by game that I make a decision, and it is about partnerships as well.
"He [Vermaelen] has been a substitute for quite a few games and he still plays a big part in the game. Koscielny was a long, long time during the season in that position and that can change very quickly."