The issue with this figure is that it comes from the Census Bureau, and their definition is broad and simple that it doesn't into account for example. Here's the definition they use:

Vacant Housing Units. A housing unit is vacant if no one is living in it at the time of the interview, unless its occupants are only temporarily absent. In addition, a vacant unit may be one which is entirely occupied by persons who have a usual residence elsewhere. New units not yet occupied are classified as vacant housing units 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, or if there is positive evidence (such as a sign on the house or block) that the unit is to be demolished or is condemned. Also excluded are quarters being used entirely for nonresidential purposes, such as a store or an office, or quarters used for the storage of business supplies or inventory, machinery, or agricultural products. Vacant sleeping rooms in lodging houses, transient accommodations, barracks, and other quarters not defined as housing units are not included in the statistics in this report. (See section on "Housing Unit.")

https://www.census.gov/housing/hvs/definitions.pdf

As you can see this definition doesn't really take into account a lot of genuine factors. For example, a lot of units are in really poor condition and require renovations in order to be livable again, but they're counted as vacant because they still have their exteriors in place. Same goes units. They're also counting units that are not entirely completed, units that are occupied but just temporarily like vacation homes, and mobile homes. We do have a lot of vacant units in this country, but it's not as much as this figure would lead you to believe. In reality, we need new units, we need a lot of them, and we need them ASAP.