NAHB uses a vacancy target of 13%. The 15.2 million current vacancies currently put the vacancy rate at 10.9%.

I couldn't find NAHB's reasoning behind the 13%, other than it's a holdover from previous analyses.

In manufacturing, and most processes, you never want to run at 100% capacity. Rather, you aim for an average of 85%, which allows you to absorb any peaks you may encounter.

This would translate to a 15% housing vacancy rate, which is larger than even NAHB's target.

The Census Bureau publishes a vacancy rate report, and vacancies not only include units for sale and for rent, but also those already rented or sold but not occupied (940,000), second and/or vacation homes (3,200,000), and "other", which includes AirBnB/VRBO homes (3,783,000).

https://www.census.gov/housing/hvs/index.html

That leaves only 3,179,000 units for rent and a mere 735,000 for sale, totaling less than 4 million livable units in the entire country.

4 million is far less than the 15.2 million you are interpreting as "available". Of the 3.8 million they are suggesting need to be built, only 950,000 would be available for rent or purchase.