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

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 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”).