Although I think Vermaelen's TOP CLASS I do think Gary Cahill is a better 'defender' than both Koscielney and Djourou. We conceded 43 league goals last year, I think its just crazy to assume the defenders we have are good enough. Fans from other clubs dont (like David Scofield) seem to share that view.
Vermaelen was team of the season in his first season. He's also younger the Cahill. I think I'd rate Vermaelen above him when fully fit. Not sure about Djourou - he was fantastic for most of last season but trailed off badly towards the end. I think he will be great in years to come, but needs better stamina.