Does Vex - Apathetic stun an enemy unit played anywhere on the board when Vex holds a battlefield?

To answer your question regarding Vex, Apathetic:

Ruling: Yes, Vex, Apathetic will stun an enemy unit played anywhere on the board, provided Vex is currently at a battlefield.

Reasoning

According to the card text for Vex, Apathetic:

"[Deflect] When an opponent plays a unit while I'm at a battlefield, [Stun] it. They can't move it this turn."

1. Trigger Condition: The ability triggers "When an opponent plays a unit." It does not specify a location restriction for where that unit is played.
2. Vex's Requirement: The only condition for the ability to be active is that Vex must be "at a battlefield."
3. Effect: If Vex is at a battlefield when an opponent plays a unit anywhere on the board, the trigger condition is met, and the effect (Stun) will resolve on that unit.

Chain and Timing

• Triggered Ability: Because this is a "When..." ability, it is a Triggered Ability (Rule 376).
• Chain: When the opponent plays a unit, the unit enters the board, and Vex's triggered ability is placed on the chain as a pending item (Rule 376.3).
• Resolution: Once the chain resolves, the unit played by your opponent will be stunned and gain the restriction that it cannot move this turn.

Note: I could not fully verify this ruling against the retrieved rules.