Voting on already-existing challenges leaves a bitter aftertaste. Your vote is motivation enough for users to participate in the challenges, which are by far the biggest motivation of all.
It's also beneficial to vote for articles you like regardless of what the community mods decide is best. This makes your votes more inclusive. Until each competition’s results have been assessed, withhold these extra votes. This way, participants remain equally positioned for the competition.
An outsider looking in sees this note to you as someone who has never officially partaken in the Engagement Challenge.