My guess is that movement in the existing home inventory is a decent proxy total vacancies.

FORBES: Housing in 2013

Each month, he and colleagues survey 4, 000 real estate agents across the U.S. The topics: what kind of foot traffic those agents see, the price movement of listed properties, incentives offered by home builders, overall inventory of homes on the market and the amount of time needed to close deals.

FORBES: Magazine Article

Teicher notes the liquidation of Borders in 2011, but also credits the ongoing "buy local" movement and independents' growing comfort with modern technology, whether for more efficient inventory systems or more effective online promotion.

WSJ: Book industry gathers for annual convention