Unfortunately it isn't actually that straightforward. That number includes abandoned and run down homes that are currently unlivable, houses that aren't actually on the market because they're being remodeled, they exist in the middle of nowhere where people don't want to live, etc. Fundamentally, the problem with housing in the US is supply. We don't build enough housing in the places people want to live.
While on the topic, a lot of people say that housing is commodified and that's why it sucks. This is not accurate. Housing is treated as an investment that should go up in value over time, not a commodity that can be easily bought, sold, and traded.
If anybody is interested in learning more about housing in the United States from someone who studies this full time, I recommend Clayton Becker
To my understanding, vacant housing refers to housing in which someone is not currently living, including housing that is under repair/renovation, needs repairs, or is abandoned/condemned.
https://darrellowens.substack.com/p/census-reveals-vacant-housing-mysteries
Also, of the housing that is vacant, two thirds are vacant for 6 or fewer months, and ~85% are vacant for 12 or fewer months. The median duration of vacancy is about 2.4 months. Only a small portion of vacant housing is actually vacant in the way people typically think when they hear vacant housing. Freeing up the ~10% of housing that is actually vacant long-term the way people think just would not alleviate the housing crisis. We gotta build more housing, y'all.
https://www.tiktok.com/@divasunglasses/video/7189814160165702955