Dear Mr. Shafer,
Thank you for posting your request on History Hub!
Casualty lists for the Vietnam War are in the custody of the Electronic Records Division (RDE) and are available via AAD (Access to Archival Databases) on the National Archives website at: https://aad.archives.gov/aad/. Please click on “casualties” under the category section and look for the Cofelt Database that is searchable by unit.
We searched the National Archives Catalog and located a series titled Unit Records, 1956 - 7/1976 in the Records of the U.S. Forces in Southeast Asia (Record Group 472) that includes organizational histories and general orders of the 809th Engineer Battalion (Boxes 13-19). We also located the Operational Reports-Lessons Learned (ORLLS) of Units and Organizations, 1964 - 1973 in the Records of the Army Staff (Record Group 319) that include ORLLs of the 809th (Boxes 224-225); and a series titled Organizational History Files, 1959 - 1973 in the Records of the U.S. Army, Pacific (Record Group 550) that includes 3 file units of the 809th Engineer Battalion. For access to and/or copies, please contact the National Archives at College Park - Textual Reference (RDT2) via email at email@example.com.
We hope this is helpful. Best of luck with your research!