There's a reason why Vermaelen is shafted to left back for Belgium and Vertonghen gets the central role with Kompany.
Are you sure it's not more because Vertonghen is a shit left back? At the prices being quoted and the age he's at, it'd be a miracle if this guy could really play across the left of the defence and at DM at Premier League level.
Seems more likely that Vertonghen is as average at left back as Vermaelen is, and possibly not much chop at DM either.