I think it would populate the servers more if the community were playing maps which the majority choose because that's where people enjoy playing the most.
The voting process should be triggered
A) once a team achieves 90 points
(B) once the game finishes
If A, then the vote should be a scroll-wheel option so that it is easily accessible to players yet also out of the way enough that players aren't distracted by it, such would be the case with a menu popping up.
It could be a chat option, but with everyone constantly killing each other it would most likely be spammed out of sight from players who weren't quick to look. However you could have an info box appear in the top of the screen saying "Type "!vote <syrta/sofia/neochori/etc..>" for the next map"
Alternatively, you could also use ARMA's in-game command menu to vote through the use of triggers.
If possible, going with B would allow for a redesign to the end-game screen (showing your points on the left of the screen and then a Vote Map box which would pop up at the end of the game only. This section would be on the screen and clickable, displaying how many people have voted for what map.
This would likely take a lot of time to script, and therefor A is the easier alternative.