How about adding a silver version of vox populi badge?
That should encourage people to vote more.
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I do not find it useful. Vox Populi is for one time badge. Making it silver version = Used 40 votes for 100 days, which may cause random or otherwise mindless voting just to earn the badge.
Silver badges are awarded for longer term goals. Silver badges are uncommon, but definitely attainable if you’re interested. (from SO badges page, written under "Legend" caption)
Neither making it silver increases knowledge.