According to the U.S. Census Bureau, unoccupied new housing units are classified as vacant if:

Construction has reached a point where all exterior windows and doors are installed and final usable floors are in place. Vacant units are excluded if they are exposed to the elements, that is, if the roof, walls, windows, or doors no longer protect the interior from the elements.

For this and other definitions as of 2008 Q4, please see this official document: ( http://www.census.gov/hhes/www/housing/hvs/qtr408/q408def.html ).