Yeah see image below, Vermaelen was already running away from goal long before (maybe it was his man but he was long gone), Mertersacker was the only one who could see the attacker and therefore really should have covered him rather than ignoring him. Both at fault but ultimately Mertersacker should have covered it IMO:
http://s11.postimage.org/fkmmzg4nn/mertersacker.jpg
Since we play zonal marking, no player marks any particular man and thus Vermaelen was simply covering his area, the player behind Mertersacker would have covered the other guy.
Ultimately though it was a poor performance by everyone IMO, we conceded two goals and didn't score any which is hardly a strong case for either defence or attack.