Trinh Xuan Thanh, accused in Vietnam of mismanagement at a subsidiary of national oil and gas company PetroVietnam, disappeared in Berlin in July. Shortly after, Germany ordered Vietnam's intelligence attache to leave the country.
Vietnamese authorities say the 51-year-old turned himself in to police in his homeland in August.
German Foreign Ministry spokesman Rainer Breul says Vietnam's ambassador was informed yesterday that a second diplomat had to leave the country because there was "evidence that he was involved in this incident."