Does mirror image token get stunned if enemy Vex Apathetic is on a battlefield?

Yes, the Reflection token created by Mirror Image will be stunned if your opponent has Vex, Apathetic at a battlefield.

Reasoning

1. Trigger Condition: Vex, Apathetic has the ability: "When an opponent plays a unit while I'm at a battlefield, [Stun] it."
2. Playing a Unit: Mirror Image explicitly states: "Play a ready Reflection unit token to your base." Because the effect uses the word "play," it satisfies the trigger condition for Vex's ability.
3. Interaction: As established in RiftJudge FAQ #8758, tokens created via effects that use the word "play" trigger Vex, Apathetic's ability. Since the Reflection token is being "played" to your base, Vex's ability will trigger and stun that token.