San Francisco — Can't find an apartment? No wonder. Statistics from a survey made by US Housing Markets show some 160,000 rental units disappeared from the market in the year prior to June 1981. The US Census Bureau reported a 5 percent drop in the rental vacancy rate, the lowest since the 1940s. Seventy-five percent of the new nonvacancies were in the Southern states, with severe tightening in the Houston area.