Depression is possible as a result of your experience in Vietnam “changing your nature and outlook on life” but this question is difficult to answer with so few details. You did not detail the ways in which you believe this experience changed your world view or contributed to your depression. Theoretically, if you arrived in Vietnam believing certain things about yourself and the world and this experience negatively altered your view on these matters, it is possible that your time at war contributed to your depression. Without more information, however, I am unable to say with any confidence how, why, or if your Vietnam experiences contributed to your depression.
You mentioned that you have anxiety and panic in addition to depression. Many Vietnam veterans experience anxiety and panic, years after their service. Often times it is thought that these panic and anxiety attacks are a result of their experiences in war and they are subsequently diagnosed with post traumatic stress disorder (PTSD). PTSD is one of the most common disorders that individuals who have fought in a war suffer from. Even if an individual “comes to terms” with their time at war, he or she may still experience panic or anxiety attacks.
If you would like a more detailed answer, please consider writing back and providing me with more information. It would be helpful, for instance, to know how you think your current depression symptoms may have been related to your time in Vietnam. In your mind, it seems that you have made this connection and it would be helpful if you could elaborate on this further. I would be glad to write a more complete answer for you if you provide more specific information regarding your question. Thank you for your service to our country.