Even on main commercial corridors in the city, long-empty storefronts have raised concerns about blight from merchants, customers and politicians. In response, Supervisor Aaron Peskin drafted a vacancy tax measure that now appears on the March ballot as Proposition D. Lee Hepner, a legislative aide to Peskin, explains how the proposed tax is designed to motivate landlords to rent properties, how it could stabilize existing small businesses, and how the city will know if it’s working.