You are right about the price, but I believe Vertonghen won't need much adapting to Spanish football. He already played at a side (Ajax) that played posession-based football, building from the back and with a high defensive line. Spanish football, especially Barca, is not much different except the level of players is higher. But that he has already encountered in England.
But probably he will cost around 25M, so I guess we won't pay that. Mark my words; we will be stuck with a mediocre CB to not spend too much...