We have more vacant units than homeless residents in virtually every metropolis and rural backwater in the country. I'll spot you its oversimplified, as there's more to housing than just the physical structure.

It's oversimplified because there are a number of reasons why a unit might be vacant at any one time. A lot of units counted as vacant are simply between occupants, many more are derelict and not suitable for human habitation. You might be able to get homeless people into those units faster under socialism, but the talking point also the housing crisis is limited to solving homelessness, when it's much larger than that. You need a solution that solves the whole problem, not just one facet of it.

But the YIMBY plan to just "build more build more build more" completely neglects this core truth. We build units to incentivize new consumption and new financial investment, not to shelter an existing homeless population.

That's why I specifically mentioned PUBLIC housing. If the subject of this thread is about what policies leftists should support and what kind of housing policy socialism should deliver, then I'm saying a policy of building lots and lots of PUBLIC, i.e. NOT commodified, NOT for profit, housing is the proper solution to the housing crisis. And there's dozens of more reasons why densifying American cities and suburbs is good policy- the SFH home suburban development model America has chosen is an environmental, economic, and social disaster, and ought to be remedied at all costs.