Which KPI is most likely to be a vanity metric?

Reach can become a vanity metric when it shows audience exposure without proving meaningful action or business impact. In Amazon Ads terminology, KPIs should connect measurement to the campaign objective being evaluated. A large audience number may look strong, but it does not show whether people engaged, purchased, returned, or became more loyal. Reach is useful for awareness goals, but it is weaker when used alone to judge performance.