Reimer was promoted to temporary major in September 1968 and permanent captain in June 1969. He served as executive officer and S–3, 2d Battalion, 4th Artillery, 9th Infantry Division, United States Army Vietnam from 1968 to 1970 and as an instructor at the United States Army Field Artillery School at Fort Sill in 1970. Reimer attended the Command and General Staff College from 1970 to 1971, followed by an assignment as personnel management officer, Assignment Section, Field Artillery Branch, Office of Personnel Operations in Washington, D.C.