I am attacking a battlefield, I attack with a 2 might unit. The opponent reacts with gust, I react with vi's ambush ability, would vi be able to be played on this battlefield?

Yes, you can play Vi, Peacekeeper to the battlefield as a reaction, and she will successfully enter the battlefield before the chain resolves.

Here is the breakdown of the interaction based on the official rules and timing:

1. The Chain Sequence

1. Attack: You attack with your 2-might unit. The battlefield becomes contested (Combat begins).
2. Opponent's Action: Opponent plays Gust targeting your 2-might unit.
3. Your Reaction: You play Vi, Peacekeeper via her [Ambush] ability.

2. Resolution and Timing

• Vi Enters Play: As per RiftJudge FAQ #9129, units enter the battlefield immediately when played. Even though you are in a "Closed" state (due to the chain containing Gust), Ambush allows you to play the unit as a [Reaction]. Vi enters the battlefield immediately and finalizes.
• Trigger Placement: Once Vi is on the battlefield and combat is ongoing, she gains the "Attacker" designation. Her "When I attack" triggered ability then triggers and is placed on top of the chain (Rule 383.4.e.2).

The current state of the chain (bottom to top):

1. Gust
2. Vi’s "When I attack" trigger

3. Resolution (LIFO)

According to Rule 340.1 and the LIFO (Last-In, First-Out) principle, the topmost item resolves first:

1. Vi’s Trigger Resolves: You choose an enemy unit at the battlefield to [Stun].
2. Gust Resolves: After Vi's trigger finishes, the next item on the chain, Gust, resolves. It checks for its target (the 2-might unit). If the unit is still on the battlefield, Gust returns it to your hand.

Summary

• Can you play Vi? Yes, Ambush allows you to play her as a [Reaction] to the battlefield where you have units (the 2-might attacker).
• Order: Vi enters the battlefield first (finalizing immediately), then her triggered ability is added to the chain on top of Gust, and finally, the chain resolves in the order of Vi's trigger followed by Gust.

This is my interpretation based on Rule 340.1 (LIFO resolution), Rule 822 (Ambush timing), and the established rules for chain item resolution. No official FAQ exists for this exact stack, but it follows the standard mechanics for triggered abilities and reaction timing.