If I'm at a battlefield with Hwei and Moonfall and an enemy Vi Peacekeeper into that battlefield, who is the attacker and who is the defender?

When you use Moonfall to move an enemy Vi, Peacekeeper into a battlefield you already control (where your Hwei is present), the resulting combat designations are as follows:

• Attacker: Your opponent (the controller of Vi, Peacekeeper).
• Defender: You (the controller of the battlefield).

Reasoning

According to RiftJudge FAQ #5394 and FAQ #2395, when you use a spell or ability to move an enemy unit into a battlefield you already occupy or control, that enemy unit is the one applying the Contested status to the battlefield.

Per Rule 459.2.b.1, the player whose unit(s) applied the Contested status to the battlefield is designated the Attacker. Because Vi, Peacekeeper is the unit that contested your battlefield, her controller becomes the Attacker, and you become the Defender.

Sequence of Events

1. Casting Moonfall: You play Moonfall, choosing the battlefield where Hwei is located and targeting the enemy Vi, Peacekeeper.
2. Move and Contest: As Moonfall resolves, Vi, Peacekeeper is moved to your battlefield. By moving into your controlled/occupied battlefield, Vi applies the Contested status.
3. Combat Designation: Because Vi's entry applied the Contested status, the game designates her controller as the Attacker and you as the Defender (Rule 459.2.b.1 and 459.2.b.2).
4. Triggers: Vi, Peacekeeper's "When I attack" ability will trigger, as she has now gained the Attacker designation (per FAQ #10437).

This interpretation aligns with the established Riftbound mechanics that distinguish between moving into an open/uncontrolled battlefield (where you apply contested status yourself and become the attacker) and moving an enemy into an already occupied battlefield (where the enemy unit you moved becomes the attacker).