Now, I know more about WW2 than the Cold War so bear with me, In Korea, there was a UN coalition, if I'm not mistaken. But I've never read anything about a UN force in Vietnam. I understand why there wasn't any help later in the war but why didn't any other nation send troops to Vietnam?
The USSR would have veteod a Security Council resolution for a UN force in Vietnam. This was possible during the Korean War because the USSR was boycotting the UN at the time. Other countries supporting South Vietnam were members of the South East Asia Treaty Organization which included South Korea, Australia, the Philippines, New Zealand, Thailand, and Taiwan.