About Vertonghen: He has a lot of infiltration power, reads the game well, which helps him make up for the lack of speed (though he's not that slow either) and he has a killer long shot in his left foot as well.
If he makes a mistake in defense from time to time, it's mostly because he thinks rather offensively. He's gotten a lot of criticism at the start of the season, but to be fair: he got a lot MORE than he deserved (meaning some other defenders were spared, due to the fact they're even younger than him + the midfield collapsed more often than not leaving the defense exposed.)
So is he world class? No, I don't think so.
Is he another Kompany? Not likely...
Does he have the potential to become a trustworthy CB or LB?
Yes, imo everything is there, but it's up to him to make it happen.
Besides, with the way Lescott and Kompany are playing right now, it's not exactly a gift to try and earn your spot. So maybe he should sign with another club for his own development.