WASHINGTON, May 18 (Reuters) - Federal Reserve Vice Chair Randal Quarles will tell lawmakers a strong economic recovery is underway in the United States, but is not yet complete. In prepared testimony published on Tuesday, Quarles said some households and businesses remain vulnerable, although the broader economy is in the “last stretch of the return to normal.” (Reporting by Pete Schroeder; editing by Jonathan Oatis) Our Standards: The Thomson Reuters Trust Principles.