If Rengar pulls my Vex Apathetic to a battlefield with Irresistible Faefolk, will their ambush units be stunned if played right after?

To determine if your Ambush units will be stunned when played, we must analyze the interaction between the Vex, Apathetic trigger and the timing of playing a unit via Ambush.

The Ruling

Yes, if you play an Ambush unit while Vex, Apathetic is present at the battlefield, your unit will be stunned.

According to RiftJudge FAQ #9735 and #9240, Vex, Apathetic triggers its ability—"When an opponent plays a unit while I'm at a battlefield, [Stun] it"—whenever an opponent plays a unit, regardless of the method used to play it (including Ambush).

Sequence of Events

1. Preparation: Irresistible Faefolk is at the battlefield and has moved Vex, Apathetic to that same battlefield.
2. Reaction Timing: Because Vex is now present at the battlefield, the game state is Closed (due to the presence of units from both players). You may play an Ambush unit as a [Reaction] to this state.
3. Trigger: As soon as you play your Ambush unit, it enters the board. Vex, Apathetic's triggered ability condition ("When an opponent plays a unit while I'm at a battlefield") is met.
4. Chain Placement: Your unit's entry into play and Vex's triggered ability are both placed on the chain.
5. Resolution: Per the chain rules (LIFO), Vex's ability resolves. Your Ambush unit is Stunned and gains the restriction that it cannot move for the rest of the turn.

Summary of Mechanics

• Trigger Independence: The stun is not contingent on how the unit arrived at the battlefield, only that it was "played" while Vex was present at a battlefield.
• Timing: Even though you are using a [Reaction] to play the unit, the act of "playing a unit" still satisfies the specific trigger condition for Vex, Apathetic.
• Stun Impact: Per Rule 423.1.b, a stunned unit does not contribute its Might to damage in the combat damage step, which will significantly weaken the impact of your Ambush unit in the ensuing combat.

This interpretation is based on the standard chain and triggered ability rules, as well as specific FAQs regarding Vex, Apathetic's interaction with units played via Ambush.