I have a Sprite at bf.. Opponent plays in Noxus Hopeful and Vi Peacekeeper.. I react to Vi by playing Janna Savior. Will the Sprite be stunned when Janna chooses to move Vi back to base? Or no stun since Janna healed or also no since Vi was in base at end of resolution?

To determine if the Sprite is stunned, we must look at the timing and the "here" constraint of Vi, Peacekeeper's triggered ability.

The Ruling

No, the Sprite will not be stunned.

Explanation of the Sequence

1. Vi enters the battlefield: When Vi, Peacekeeper enters the battlefield and joins combat, she is designated as an attacker. This triggers her "When I attack" ability, which is placed on the chain.
2. Vi's Trigger: The ability reads: "When I attack, [Stun] an enemy unit here."
3. Reaction: You play Janna, Savior in response to Vi's trigger.
4. Chain Resolution (LIFO - Last-In, First-Out):
   • Janna, Savior resolves first: Her ability executes: she heals your units at that battlefield and moves Vi, Peacekeeper to her owner's base.
   • Vi's Trigger attempts to resolve: The ability attempts to execute its instruction to "[Stun] an enemy unit here."
5. Failure: Because the term "here" requires the source of the ability (Vi, Peacekeeper) to be present at the battlefield for the effect to resolve (per RiftJudge FAQ #10045 and #10627), and she was moved to base by Janna, the ability fails to find its source and the stun does not occur.

Summary

The stun fails because the source of the ability (Vi) is no longer present at the battlefield when the trigger attempts to resolve. Whether or not Janna performed a heal is irrelevant; the governing factor is that Vi was removed from the battlefield, causing her "here" condition to fail.