VDFs don't replace proof of work. In essence, POW is a lottery where everyone gets their own random lottery ticket and the lowest number wins. VDFs, instead, allow a large group of people to all share a SINGLE ticket, which simply spits out the ID of the winner.

This is conceptually a completely different approach and VDFs therefore aren't susceptible to the same problems as POW (though they have other unrelated problems, which this contest is trying to mitigate)