The Vox Populi badge is described as :"Use the maximum 40 votes in a day".
However I tried to get this one a couple of times now and I never got to more than 31 votes. Then I get the message "You have used all your votes for today".
It looks like a bug to me or there should be a requirement to have "XX question votes up/down votes" or a rep of ZZ to get to 40 votes?
As to the "usefulness of the badge"
Well ... how useful are the badges anyway :-s.
I see a benefit in "appreciating other peoples effort" by upvoting answers and questions (and somehow in downvoting as well). After all this is a questions and answers site and not a discussion forum.
For those willing to give it a try. It is quite some work to find 40 questions/answers worthy to upvote in a day.