From what I can gather, the voting thing is not explained. Nobody knows, just assumptions from people reading far too much into it.
Theories include;
The order of which they are unlocked via milestones (as willow says)
The order they will just be given out to everyone, with no pre-requirement to gain any achievements
Least popular one will be replaced with a different unlockable
The whole thing is a lie and it is actually a Spy update
/me is glad I'm banned from the steam forums now.