Can Vi's ability be used when defending in a showdown declared by the opponent?

Ruling: No, Vi's ability cannot be used during a showdown. Vi's ability does not have the "Action" or "Reaction" keywords, so it can only be used during your own turn, outside of a showdown, and only while no chain currently exists.

Nuances:

• The ability must be used during your own turn
• It cannot be used while any chain exists
• It cannot be used during any showdown (whether attacking or defending)