Can Vex Apathetic ability/trigger be missed?

Yes, the triggered ability of Vex, Apathetic can be missed.

According to Tournament Rules Section 702.2 (Missed Trigger), a triggered ability is considered "missed" if the accountable player (the controller of Vex, Apathetic) does not demonstrate awareness of the trigger before the first time it would have an observable impact on the game.

Key points regarding missing Vex's trigger:

• Observable Impact: Per Rule 506.3.d.6 and 506.3.d.7, the "observable impact" of Vex's ability includes the unit becoming [Stun]ned or the unit being restricted from moving. If the opponent plays a unit and you fail to acknowledge your Vex trigger before that unit attempts to act or move in a way that would be prevented by the [Stun]/restriction, the trigger is considered missed.
• Penalty: Per Rule 702.2, "Missed Trigger" generally carries [No Penalty] at most levels of play, provided it was not missed intentionally.
• Remedy: Per Rule 702.2.f.2, if a mandatory trigger is missed, the next opponent in turn order chooses whether to resolve the trigger immediately or have it missed altogether.
• Intentionality: If a player intentionally ignores a mandatory trigger because it is disadvantageous to them, this is considered a [Warning] rather than a "No Penalty" missed trigger (Rule 702.2.e).

In summary, Vex's ability follows standard game rules for triggered abilities, and like any other triggered ability, it can be missed if not acknowledged by its controller in a timely manner.