As with any stats-laden post, I wanted to understand what exactly a “vacation house” was. A footnote in the article references a “census of housing” and I found information on the US Census website. There are 6 main categories for vacant houses (vacation/seasonal is one of them), and at least some of them have subcategories.
It is pretty likely that “vacation house” doesn’t include a lot of housing that most people would think of when they think about vacation houses. If something is listed on AirBnb or VRBO, for example, it is almost certainly not included in these stats.
If the home listed on Airbnb is also occasionally used by the owner, it would count as a vacation home. Otherwise it's just business property, more similar to a hotel than a vacation home. I got the sense that it's more about vacationing use by the owner, not short-term rental use by a non-owner.
It is pretty likely that “vacation house” doesn’t include a lot of housing that most people would think of when they think about vacation houses. If something is listed on AirBnb or VRBO, for example, it is almost certainly not included in these stats.