Sometimes it takes time to appear, but I expect that if that were your issue you would have it already.
What likely happened is that you did not meet the criteria for the badge (using the maximum possible 40 votes in one day), but still used all of your votes. You get 30 total votes per day, with an additional 10 votes that can be used on questions. The way these 10 votes works, though, is tricky. You need to use them early in the day. Once you get the countdown on your votes telling you how many you have left for the day, it's too late (but you can then determine the total you are able to cast that day). In order to get the Vox Populi badge, you need to use 40 votes, not just all of your votes.
Some explanation is available on Meta, with a bit more detail here. I'm not sure I'm 100% clear on how it works, but I do know that if you want to get the full 40 votes in a day, you need to vote on questions early.