The optimal vacancy rate isn't zero, renters (and relocating homeowners) need to be able to move and to reject bad landlords.
Speculation driven rises in residential real estate prices buy/rent are a plague on society and we need to stop framing the problem so that we lose sight of the purpose of housing, it’s to house people…
What's the right period of vacancy to allow for 1, 2, 3, and 4-bedroom units as a function of floor level, direction faced, time of year, elevator or not, age of building, distance to school and transit, etc?
The unit can provide utility as long as it is fit to live in. The only question is what price people are willing to pay for it.
We can even generously set it at 1 year just to start things off. If landlords are still too comfortable keeping units empty for the sole purpose of holding out for another price, then it can be revised lower.