If Vex Apathetic is on a battlefield and I play Mirror Image, is the unit stunned?

Yes, the Reflection unit token created by Mirror Image will be stunned if your opponent controls Vex, Apathetic and that unit is located at a battlefield.

Reasoning

According to RiftJudge FAQ #9485 and #10242, this interaction functions as follows:

1. Trigger Condition: Vex, Apathetic has the triggered ability: "When an opponent plays a unit while I'm at a battlefield, [Stun] it. They can't move it this turn."
2. Playing a Unit: Mirror Image explicitly uses the word "play" in its text ("Play a ready Reflection unit token to your base"). In Riftbound, a token is considered "played" if the card effect that generates it uses that specific keyword.
3. Interaction: Since the token is being "played," it satisfies the trigger condition for Vex, Apathetic. Provided Vex, Apathetic is at a battlefield, its ability will trigger and be placed on the chain. Upon resolution, the Reflection token will be Stunned and gain the restriction that it cannot move for the rest of the turn.

As confirmed in RiftJudge FAQ #10242, this ability uses programmatic selection, so it will stun the token even if the token possesses protections that would normally prevent it from being chosen.