Does Vex Cheerless pay for Deflect as an additional cost?

Yes, when your opponent casts a spell that targets a unit with Deflect while Vex, Cheerless is in combat, the cost of that spell includes the Deflect cost as a mandatory additional cost.

According to Rule 809.1.d and RiftJudge FAQ #9237, Deflect is a mandatory additional cost. Because Vex, Cheerless affects the total cost of enemy spells (increasing it by 1 Energy and 1 Power), this increase applies to the entire cost calculation, which includes the mandatory Deflect cost.

In short:

1. The base cost of the spell is determined.
2. The mandatory Deflect cost is added to that total.
3. The cost increase from Vex, Cheerless is then applied to that total, making the spell more expensive for your opponent to cast.