Does Vex reduce the total cost of a repeat spell?

Yes, Vex can reduce the total cost of a spell with the Repeat keyword.

According to the RiftJudge FAQ #8444, Vex's ability reduces the total cost of a spell, which includes base costs, repeat costs, and mandatory additional costs (like Deflect).

Here are the key points regarding this interaction:

• Repeat is an additional cost: Repeat is treated as an optional additional cost that is added to the spell's base cost. Because Vex's discount applies to the total cost of the spell, it can reduce the cost of the Repeat portion.
• Minimum Energy Restriction: Vex's ability includes a "minimum of 1 Energy" restriction. However, this restriction applies only to the discount provided by that specific Vex unit, not to the final cost of the spell after all other modifiers (like Ezreal's cost reduction) are applied.
• Order of Operations: Players can choose the order in which they apply cost reductions. By applying Vex's discounts first (respecting its minimum) and then applying other non-restricted cost reducers (like Ezreal), it is possible to reduce the total cost of a repeated spell to 0.

This is based on the ruling in FAQ #8444. No official rule chunk contradicts this, and it is consistent with the game's rules on determining total cost (Rule 391) and applying discounts (Rule 353.4).