Interestingly, as short as it is, this post is one of my most often visited posts from Google searches. So I thought I'd update it with a little more detail about agent orange exposure and in-country service in Vietnam.
If you have diabetes and served in country it's a legal presumption that exposure to diabetes caused the diabetes. You don't have to prove the exposure and you don't have to prove the causation.
I was in Vietnam, but technically wasn't in-country. I was on a destroyer mostly on the Navy Gunline in the DMZ. We also did some close in gunfire support from DaNang Harbor and offshore at Hue right after the Tet offensive of 1968. But I never had orders to go to Vietnam, I had orders to report to USS Hull DD945, then the ship went to Vietnam.
If you served in the Brown Water Navy in Vietnam you were in-country. You had individual orders to Vietnam. If you were shipboard at Yankee Station you probably weren't in-country. You were in a war zone, but you were not close to the Vietnamise coastal waters.
That's why you see the language in my appeal about operating close to the shoreline in the DMZ and anchoring inside DaNang harbor. That put me within coastal waters which meant that I was legally presumed to have been exposed to Agent Orange and my diabetes was presumed to have been caused by that exposure.
You have to play the game, and you have to know the rules of the game.