Can Vi, Peacekeeper stun Baron Nashor with her 'When I attack' ability?

Ruling: No. Vi, Peacekeeper's ability requires you to choose an enemy unit, which makes it a targeted ability. Baron Nashor's protection ("I can't be chosen by enemy spells and abilities") prevents him from being a legal target for Vi's stun effect.

Nuances:

• Even if Baron Nashor is the only enemy unit at the battlefield, he still cannot be chosen as a target, and Vi's ability will fail to find a legal target.
• This differs from programmatic selection effects (like Vex, Apathetic's "stun it") which automatically identify a specific unit without requiring a choice and can bypass Baron's protection.