Might have something to do with the fact that Vermaelen never played centreback for Ajax.
Vertonghen plays in a two bob League and has done next to nothing in his career. Vermaelen plays for one of the biggest clubs in Europe, he's played in Champions League semi-finals, been named in the Team of the Year in the biggest and best League in the world, and is generally considered one of the best defenders in the League.
Maybe Vertonghen has the potential to be a better player in the future, but right now it's not even close. Why do you think Spurs are on the verge of signing him?