In terms of pure numbers, I think Vietnam can be counted as one of the least deadly wars we have fought. The Civil War and World War II are definitely higher up on the fatality charts. But I don’t think the OP adequately addresses the possibility of death by other means - not death in combat, but death due to faulty technology or personnel troubles.
My father served in Vietnam (enlisted in the Navy because he thought his number was up), and from what he’s described, it was no cakewalk for the low-risk personnel involved. (I’ll define low-risk personnel as branches of the military that didn’t see a lot of action, like the Navy.)
He was on a supply ship cruising the Vietnam coast as a refueling center for the vessels that actually saw battle. But as I recall, it was still dangerous on those ships, as you never really knew what would happen next.
He once told me of an incident involving some mini-helicopters. My memory of this story really is a bit shady, but I think that the small helicopters were operated by remote, and were outfitted with lots of cameras to spy on the North Vietnamese Army. Several were given to Dad’s ship to deploy, but he said that from the outset there were difficulties. They soon realized that as soon as the spy-choppers got up in the air, most of them stopped responding to the radio signals. They’d just fly off into the wild blue yonder.
The captain of the ship did NOT like this, and whenever this happened, would order that the ship chase it down (I think to recapture it and prevent it from falling into enemy hands, so that they couldn’t learn anything from the technology).
There was also a contingent of Marines on the ship, and they came across one of the mini-helicopters dead in the water - it apparently fell in after running out of gas. The captain was elsewhere occupied or just plain busy, and wasn’t around when the chopper was reclaimed. Instead of notify the captain, the Officer on Deck sent a Marine overboard with a rope tied around his waist, and had him do something to secure the chopper to the ship. (This part has me mystified. I can’t recall if the Marine was supposed to attach it to a harnass so it could be lifted on deck, or if it was supposed to be attached to his rope and lifted in, or…)
Anyways, the Marine was still working on the spy-chopper when a shift change occurred on deck. Some miscommunication happened or signals got otherwise crossed, and the new Deck Officer wasn’t informed that the ship was attached by rope to a man over the side. And since he didn’t know, and no one else on the new shift knew (that part seems a bit weird), he wondered why the ship wasn’t moving.
He ordered the engines back to work.
Thankfully, the Marine had a knife on him, and sawed through the rope before being dragged to a watery death. But he wasn’t very happy about being left alone for several hours before they figured out that he was still back there.