There absolutely is a "normal" amount of vacancies because it takes time for a house to sell, for someone to move out and then someone else to move in, for a new lease to be signed, etc. It's exactly the same dynamic with employment; there is a "normal" amount of a unemployment (a low single digit number) that is impossible to improve past, simply because it takes time to find a job. Housing is no different.

Also, most of the homeless are unhoused because they either can't afford a home or they have mental illness/drug addiction issues that makes them incapable of earning money in order to be able to afford a house. It doesn't matter if housing is vacant if you don't have the means to pay the rent, or if they won't even consider you for a lease anyway because you don't have a reliable source of income.