The origin of the 30,000 number is unclear, and I can't tell what kind of vacancy they're talking about - due to renovation? Fights over ownership? Transition of ownership? Intermittent use because the owners live somewhere else also? It also seems neither here nor there. SF is not a military base. The city doesn't own most of the housing here, unless those are 30,000 public housing apartments, how can the city do anything about whether they are rented or occupied? Supposing they could do something about those vacant units, what does this private development have to do with the city developing other places?
More housing at any price level will bring prices down at all levels (if the number of people stays constant) and help keep prices from rising if more people are constantly moving in.
Suppose you have 45 people in a little toy city that can spend $10,000/ mo on an apt, and 40 apartments that are really super nice. 40 rich people will get super duper nice apartments, and 5 rich people will overpay (because the landlord knows he can charge it) for a merely very nice apartment. Now suppose a developer builds 5 new $10,000/ mo apartments. Now all of the people willing to pay $10,000/ mo have super duper nice apts, and the owners of those merely very nice apartments will be forced to charge $8,000. Notice, that 5 $10,000 apts were built, but what we have is 5 new $8,000/mo apts, because the highest end of the market was oversubscribed. But it doesn't have to stop there. Maybe before the new apts were built there were 35 people willing to spend $8,000, but only 28 very nice apts, so 7 people were paying $8,000/ mo for merely nice apartments. Now that there are 5 new $8,000 apartments, only 2 people will be overpaying, and there will be 5 newly available nice apartments that those owners will have to rent for $6,000. Anyway, so on. That's now building new housing at any price level can affect the price of housing at lower levels.
Of course that won't happen if there are people that move to the city to live in these luxury apartments who would not have moved in otherwise. Like, if there are 185 people in mountain view who were not planning on moving to SF, but then hear about this apt bdg and all move into it. In that case, the new development is neutral, it does nothing.