You don't want a maximally "efficient" vacancy rate (i.e. 0), because that makes it basically impossible for people to move around - and that gives enormous power to landlords to gouge tenants.

So what vacancy rate do you want, ideally? Not sure, but you'll notice from your own source that 6.9% is around as low a vacancy rate as the US has had since the turn of the millennium.

FWIW, looking at vacancy statistics across the whole of the country can be misleading. They include houses that are vacant for any reason - including properties that are condemned, under renovation, or in the middle of nowhere where there are no jobs.

That latter point is key. As more and more economic activity has moved to white-collar coastal cities and away from rural or suburban blue-collar middle America, it's no surprise there are going to be vacancies in regions where the economy's been hollowed out. So we can conjecture that the record low vacancy rate is even more problematic, because those vacancies are likely less equally distributed to areas that need them.

From a quick spot check… yes?

Assume 120 homes in an area, evenly distributed per month on yearly leases. If all renters switch every month and it takes a month to re-rent the unit, there is a vacancy rate of 8.3%

Obviously not every renter moves every lease end, but also some units are in places no one wants to live, others are mispriced, some need renovation or extensive cleaning, etc.

But from a cursory sanity check the 6.9% number is likely reasonable for a fairly tight rental market.

Places with a higher vacancy rate are correlated with lower rents and less homelessness e.g. Indiana and Texas have a higher rental vacancy rate than California and New York (https://fred.stlouisfed.org/graph/?g=1CuFs) so whatever causes a higher vacancy rate seems to be good for affordability (not sure what you mean by “efficient”).

I was trying to explain this to my wife the other day. She was complaining that I had all these clothes on the floor in the bedroom and that she couldn't get out of bed easily because it was so messy (because I efficiently store some electronics under the clothes) but I explained that most of the house was still empty, especially by volume (since the air above the clothes is completely unoccupied). Fortunately, because I am rational and have data on my side, she relented as she usually does.

Likely a chunk of that is in cities or places in the countryside that have seen depopulation

american housing is optimized for investment, not living