If my opponent has a Vex, Apathetic at a battlefield and I attack into it with a unit and then play Vi, Peacekeeper with ambush, do I get to stun the Vex?

To determine if you can stun Vex, Apathetic with Vi, Peacekeeper in this scenario, we must analyze the order in which triggered abilities are placed on the chain and how they resolve.

Ruling

Yes, you can stun Vex, Apathetic. However, the resolution order depends on who is the active player and how the triggers are added to the chain.

According to RiftJudge FAQ #2439, when triggers occur simultaneously for both players, they are placed on the chain starting with the active player, followed by other players in turn order.

Sequence of Events

1. Attack: You move a unit into a battlefield occupied by Vex, Apathetic, initiating combat. You are the attacker.
2. Ambush: You play Vi, Peacekeeper as a [Reaction] using Ambush.
3. Entry and Triggers:
   • Vi, Peacekeeper enters the board, gaining the "Attacker" designation. This triggers her "When I attack" ability.
   • The act of playing Vi, Peacekeeper triggers Vex, Apathetic's ability: "When an opponent plays a unit while I'm at a battlefield, [Stun] it."
4. Chain Placement: Per FAQ #2439, triggers are added to the chain starting with the active player. Assuming you are the active player (the one who initiated the combat/played the unit), your trigger (Vi) is added to the chain, then your opponent's trigger (Vex) is added on top.
5. Resolution (LIFO): The chain resolves from top to bottom.
   • Vex, Apathetic's trigger resolves first, stunning your Vi, Peacekeeper.
   • Vi, Peacekeeper's trigger then resolves. Provided Vi is still on the board at that battlefield ("here"), you may choose an enemy unit—such as Vex, Apathetic—to be stunned.

Important Considerations

• "Here" Constraint: Per RiftJudge FAQ #10045, Vi, Peacekeeper must remain at the battlefield when her ability resolves. If Vex, Apathetic's ability (or any other effect) were to remove Vi from the board before her trigger resolves, her ability would fail to find its source and would not stun anything.
• Protection/Selection: Vex, Apathetic's ability uses programmatic selection, meaning it does not "target" your unit and bypasses protections that block choosing (FAQ #8772). Similarly, your ability allows you to choose an enemy unit at the battlefield.
• Stunned Status: As per Rule 423.1.a.1, a unit that is already stunned cannot be stunned again. If Vi is stunned by Vex, she remains on the board and her trigger will still attempt to resolve, allowing you to stun Vex in return.

This is my interpretation based on the standard chain rules and triggered ability priority. No official FAQ exists for the specific interaction of these two champions triggering simultaneously.