WASHINGTON — In the largest call-up of U.S. diplomats since the Vietnam War, the State Department is planning to order some of its personnel to serve at the American Embassy in Iraq because of a lack of volunteers.Those designated "prime candidates" — from 200 to 300 diplomats — will be notified Monday that they have been selected for one-year postings to fill the 40 to 50 vacancies expected next year. They will have 10 days to accept or reject the position. If not enough say yes, some will be ordered to go to Iraq and face dismissal if they refuse,...