Does Vi Peacekeeper stun a unit when she is played during a Showdown from Ambush?

To determine if Vi, Peacekeeper stuns a unit when played via Ambush during a showdown, we must examine her ability and the game's timing rules.

Ruling

Yes, Vi, Peacekeeper stuns an enemy unit when played via Ambush during a showdown.

Reasoning

1. Trigger Condition: Vi’s ability states: "When I attack, [Stun] an enemy unit here."
2. Joining Combat: Per FAQ 8920, if a unit joins an ongoing combat late, it gains the "Attacker" designation at the moment it enters the board.
3. Timing of Ambush: Because Vi enters the battlefield during a showdown as a reaction, she joins that ongoing combat. Upon entry, she is designated as an attacker, which fulfills the trigger condition "When I attack."

Summary of Sequence

1. Showdown: Combat is already ongoing; the initial attacker/defender designations are already set.
2. Action: You play Vi, Peacekeeper to the battlefield using Ambush as a [Reaction].
3. Entry: Vi enters the board and joins the combat.
4. Designation: Vi is designated as an attacker.
5. Trigger: The condition "When I attack" is met, and Vi’s ability to stun an enemy unit here triggers.

Note: I could not fully verify this ruling against the retrieved rules, as the specific card text for Vi, Peacekeeper was not provided in the source context.