When asking about Agent Orange, remember when interviewing a veteran that he may suffer from post-traumatic stress disorder and could be triggered by an interview question. Begin with questions that are less emotional in content, and slowly work up to asking harder questions regarding Agent Orange.
You could begin by asking the veteran what his position was in the Vietnam War, and eventually ask more specific questions about his involvement with Agent Orange. It may be also useful to find a comfortable location, away from sudden noises, for the interview to manage potential PTSD symptoms. As this is a sensitive topic, you can frequently check in with the veteran and ask if he is comfortable with the questions. You might also begin the interview by letting the veteran know that he does not have to answer any questions that make him uncomfortable.
It's also important to do a lot of background research on a topic before entering an interview. Make sure you know all of the publicly available factual details surrounding Agent Orange, so that the veteran doesn't need to tell you information you could easily find elsewhere. If you are using the interview for a journalism article, you may also want to create a release form, a contract between you and the veteran that clearly states how the interview will be used.