And are those vacant houses in places where people want to live?
In NYC, for example, the vacancy rate is a bit above 2%. Vacant houses in Oklahoma have zero relevance to homeless people in NYC. It also has to be considered that a lot of vacant residences are only temporary; either caught up in legal issues or briefly empty while the owners find tenants. But again, geography is critically relevant here. Vacant properties only help if they're in places where people actually want to live and where jobs are available. It doesn't matter how cheap a shack in Nebraska is if there's no job around beyond farming.