Reminder that there are not actually that many vacant properties out there where people want to live:
https://ggwash.org/view/68831/no-vacancy-is-a-problem-when-people-need-homes
Vacancy rates are very low in cities with high housing prices, and most vacancies in those cities are simply units changing hands between different residents.
As for investors buying up units, we know that they are not keeping them empty, because vacancy rates count investor owned units as well, yet have found the same very low vacancy rates. Why would investors want to lose money on an empty unit when they can simply rent out the unit instead?
Vacant units are not a solution to high housing prices.