Can Ezreal Prodigy and Vex Cheerless stack their cost reductions to play spells for 0 energy?

No, they can no longer stack to reduce spells to 0 energy. Under Rule 356.4, discounts that only affect a specific component of a cost (like Ezreal Prodigy's, which reduces the energy component) must be applied before discounts that affect the total cost (like Vex Cheerless's). Because Ezreal's component-specific discount is applied first, and then Vex's total-cost discount is applied after, the ordering prevents them from combining to fully eliminate costs the way they could before.