When I move into an occupied battlefield and start a showdown, I am the attacker. If I play Vi Peacekeeper, is she also attacking and can she stun a unit?

Yes, Vi, Peacekeeper is considered to be attacking when she enters a combat showdown, and she can stun an enemy unit.

Ruling

When you play Vi, Peacekeeper via Ambush into an ongoing combat, she joins that combat and is designated as an attacker. This triggers her "When I attack" ability, which allows you to stun an enemy unit at that battlefield.

Explanation of the Sequence

1. Combat Transition: You moved into an occupied battlefield, establishing yourself as the attacker and initiating combat.
2. Ambush as Reaction: Because you have a unit there, you meet the requirement for Ambush, allowing you to play Vi, Peacekeeper as a [Reaction] during the showdown (even if the combat chain is closed).
3. Entry and Designation: Per RiftJudge FAQ #10034, when Vi enters the board during a showdown, she joins the ongoing combat. Upon entry, she gains the "Attacker" designation.
4. Trigger: Gaining the "Attacker" designation fulfills the condition "When I attack."
5. Resolution: Vi’s ability is placed on the chain. Assuming she is not removed from the battlefield (e.g., by a reaction like Hidden Blade), her ability resolves, and you may [Stun] an enemy unit at that battlefield.

Important Considerations:

• "Here" Constraint: Per RiftJudge FAQ #10045, Vi must remain on the battlefield when her triggered ability resolves. If an opponent plays a card to kill or remove her from the battlefield in response to her trigger, the ability will fail to find its source ("here") and will not stun the target.
• Timing: As an attack trigger, this ability happens exactly once when she gains the Attacker designation. It does not re-trigger if she survives to face additional units during the Combat Damage Step (per RiftJudge FAQ #10205).