If Vi Peacekeeper attacks a battlefield and in reaction the her "when I attack, stun an enemy unit here" the opponent plays Star-Crossed targeting Vi, does the stun still resolve?

No, the stun will not resolve.

Explanation of the Sequence

1. Attack: You move Vi, Peacekeeper into an enemy battlefield, initiating a combat. Vi gains the "Attacker" designation.
2. Trigger: Vi's ability ("When I attack, [Stun] an enemy unit here") is placed on the chain.
3. Reaction: Your opponent plays Star-Crossed in response, targeting Vi, Peacekeeper and an enemy unit.
4. Chain Resolution (LIFO):
   • Star-Crossed resolves first: It returns Vi, Peacekeeper to her owner's hand.
   • Vi's Ability attempts to resolve: When it checks for its effect ("...an enemy unit here"), it fails to find its source (Vi).

According to RiftJudge FAQ #10045, the term "here" requires the unit (Vi) to be present at the battlefield when the ability resolves. Because Star-Crossed removed Vi from the battlefield, the ability fails to find its source and does not resolve.

This interpretation is based on the standard rules for targeting and source location checking (Rule 359.3.e.12, Rule 359.3.f.4). No official FAQ exists for this specific interaction, but it follows the established rulings regarding Vi's "here" constraint when the source is removed.