I'm not sure it works that way. I'm no landlord, but property tax is not subject to rent control reductions as far as I know. The primary beneficiaries of rent control are the people who are the renters, and are often able to extract value out of it from buyouts in the tens of thousands of dollars to as much as hundreds of thousands of dollars.

In a city like SF, vacancies is not a thing. Actually for that matter, in any market the market price is some equilibrium price that gives a certain vacancy rate.