They needed defender and it would be very hard to find someone better in this window. Especially that they still need some reinforcements, as Sanchez, Fabregas, Xavi, Puyol are no longer in the team.
Vermaelen is a defender thats good in technical defensive ability, but is not good at positionning. He is not very fast but has good stamina. He has average passing and good crossing.
Barcelona plays with shitloads of space in the fullbacks back, rarely actually defends or gets pressed into defending, base their entire football on short and medium passing, and dont got a single player who can challenge headers during open play.
"hard to find someone better" ?
If they are willing to spend on a good defender that suited them, they would have gotten him