In my mouse over profile summary it says I have 39 votes casted today. When I try to cast another vote (regardless if question or answer) my vote is rejected and a yellow box says my daily vote limit is reached.
Although the badge description says
Used the maximum 40 votes in a day
How I am able to earn the Vox Populi badge if I'm not able to cast the 40th vote?
You get 40 votes a day; of which 10 are question-only and 30 are for use on anything.
The unintuitive thing about how this works is you must use at all 10 question votes up before you use up the 30 anything-votes. In fact, once you reach the "only 5 votes left!" warnings (which appear when you start running out of anything-votes) your limit is set for the day, a spree of question votes then won't help.
So, the way to get to 40 votes in a single day is to vote for more questions, so that you've voted for at least 10 before the "N votes left" countdown appears.
See the following on Meta.SO for some more information and discussion:
