Dear Patrick Townsend,
Thank you for posting your question on History Hub!
If you haven't done so already, visit the National Archives Catalog to find digitized documents pertaining to the 20th Engineering Brigade in Vietnam. A precursory search using the term "20th Engineering Brigade" yielded the following: https://catalog.archives.gov/search?q=20th%20engineering%20brigade&rows=20 . You can perform a search for the 595th as well to narrow your results.
The After Action Reports are found within Record Group 472 "Records of the US Forces in Southeast Asia, 1950-1976 (https://catalog.archives.gov/id/745 ). Specifically, you're looking for the series "After Action Reports 1965-1972" ( https://catalog.archives.gov/id/305329 ). These records may not be digitized. For more information about these records, please contact the National Archives at College Park - Textual Reference (RDT2) via email at Archives2reference@nara.gov..
Due to the COVID-19 pandemic and pursuant to guidance received from the Office of Management and Budget (OMB), NARA has adjusted its normal operations to balance the need of completing its mission-critical work while also adhering to the recommended social distancing for the safety of NARA staff. As a result of this re-prioritization of activities, you may experience a delay in receiving an initial acknowledgement as well as a substantive response to your reference request from National Archives. We apologize for this inconvenience and appreciate your understanding and patience.
We hope this information id helpful. Best of luck on your research!