I was a very good boy yesterday, voting 40 times (and getting the message that told me to wait until the next day before voting again) on a good mix of questions and answers. But I didn't receive the Vox Populi badge for it. What gives?
Vox Populi requires you to use all 40 votes in a day, but you haven't voted 40 times on any site1.
Looking at your network profile, the only site where you came close was on Stack Overflow:
You are allowed to vote 40 times in a day as long as you make sure 10 of those votes are on questions before you get the warning about only have a few votes left for the day.