https://www.census.gov/housing/hvs/current/index.html
https://www.census.gov/housing/hvs/data/rates.html
Sorting by the census areas with the highest rates gets you areas like Jacksonville, FL, Baton Rouge, LA, and San Antonio-New Braunfels, TX. Superstar cities like SF and NY have rental vacancy rates of 5.5 and 5.4 respectively.