Remember that not every unit the census counts as vacant can have someone move into it. Their definition is honestly kinda weird. Some units are under construction or repair. Some are legally tied up in a divorce or estate sale. Some actually have people in them, such as non-dormitory student housing or housing for seasonal workers.
According to the census, 14.5% of vacant units for rent are vacant for less than a month, and 20.6% are vacant for more than one month but less than 2. The median vacancy has been on the market for 3.7 months, and less than 20% of vacancies have been on the market for more than 1 year.
Having a lot of units on the market for a month or two is a good thing; it means people can move to an area and find housing. You’re not going to house homeless people by sticking them into an apartment for a month or two between paying tenants.
It’s also a good thing because low vacancy rates are associated with rents going up. And the rent being too damn high increases homelessness.