Vacant homes are any home that’s not someone’s primary residence when they calculate vacancies.
That includes vacation homes, temporary housing for traveling workers or college students, houses that are sold or rented but haven’t been moved into yet, housing held up in divorce or estate proceedings, etc.
According to the census, last year there were 15 million vacant homes. Yes, that’s a lot, and yes, many can’t reasonably have a homeless person live there.