Suffrage - Used 30 votes in a day.
Vox populi - Used the maximum 40 votes in a day.
Is 40 now a new day limit for casting votes?
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You now get an extra 10 votes a day but these can only be used on questions. So to get the Vox populi badge, you will have to vote on at least 10 questions in addition to using the "normal" 30 votes.
See this blog post about voting for questions and the new 40 votes a day
See the Limits on casting votes official FAQ.
TL;DR: to safely get Vox populi, cast at most 24 answer votes and at least 16 question votes, or make sure to cast 10 question votes before casting your 25th answer vote.