Mainly because Response does not solely influence tackling and taking the ball off another player but also reading of plays and intercepting a pass. Vertonghen is a brilliant reader of play and his tackling is also better than Toby's, I feel.
I think that Vertonghen maybe reads the game a little better, i doubt it but maybe!
But intercepting and tackling, especially tackling. No way he is better then Toby. Imo. Toby isn't only better at positioning, he is a far better blocker and interceptor of the ball and tackler.
Not many people agree with me i know. Because Vertonghen looks more freatning then Alderweireld because of he's physic, people assume that he is a better defender.
But Toby concentrates and does he's job. when he doesn't play the Ajax defence looks less organised. The reason that Vertonghen is more like able here in Amsterdam is the fact that he rushes often down the center and scores a lot of 20-25M shots. Dutch favor attacking-style. And imo lack the vision (sometimes) to see when a real defender is a good player in defending.
Just an example off topic: a lot of dutch here in Amsterdam find Van der Wiel one of the best right backs in the world. Just because he can control a ball and makes forward runs. They often forget to be a world class right back you need to defend as well. Plus ofcourse deliver good forward balls witch imo he doesn't.