U.S. office vacancy rate was unchanged at 16.6 percent in the third quarter, according to real estate research firm Reis Inc.: (Reuters) - U.S. office vacancy rate was unchanged at 16.6 percent in the third quarter, according to real estate research firm Reis Inc (REIS.O). Overall net absorption, measured in terms of available office space sold in the market during a certain time period, dropped to 3.5 million square feet of office space in the third quarter, compared with 5.5 million square feet, a year earlier.