This whole CBs with the ability to play fullback bit is slightly dodgy at our level.
Vermaelen came to Arsenal promoted as a CB with the ability to play at left back, but he really can't play that position well enough at Premier League level to be a true alternative. I suspect, though can't be sure that the same would be true of Vertonghen.
There just aren't many players at all who combine the aggression and physical intent required of a top central defender with the athleticism and delivery of a top fullback, who also have the intelligence to chop and change from match to match.