I have voted 30 times today, which is the maximum. I have used my daily quota.
But for me, the Vox Populi badge shows the progress bar 30/40 — the badge is awarded after 40 votes all on one day. Is this a bug or am I missing something somewhere?
You have to vote more on questions, rather than answers. If you vote only on answers, you have 30 votes per day; each vote on a question (as long as you have five or more votes left) will increase that number by one, up to 40. There's a friendly reminder if you vote (what the system deems to be) too much on answers alone.
More details here on Meta Stack Exchange.