They vote but I think in Barça (don't know about other clubs) it's just a formality when it comes to (first) captain and vice-captain. Captaincy is based more on seniority these days and in our case also on homegrown status. It was clear when Puyol left that Xavi will take the armand and after him it was Iniesta's turn. Based on his quiet personality I don't think he was ever very vocal in the locker room but he had respect of the teammates, opponents and referees. Now it's similar with Messi. It would look strange if not even bad if team mates wouldn't vote for him.
Voting is really only deciding about 3rd or 4th captain which can become more important in future if captain/vice-captain leave and players usually move up the captaincy ranks if they don't leave before. So this season players will vote for a new 4th captain (probably between Raki and MATS) while Busi and Pique will become vice-captain and 2nd vice-captain. Don't really like either of them in those roles, especially Pique but it is what it is.