Did the British Empire during the Revolutinary War have more advantages than the United States during the Vietnam War?
Great question. These were very similar situations and I always compare them when I teach about Vietnam. I would argue that the British had a few more advantages than the US had.
First, I would argue that the British had the advantage of not being as democratic as the US would be and in not being a democracy with television. The televised nature of the Vietnam War was a major reason for the decline in American public support for the war. Since the US was such a democratic country, this decline in support put major pressure on the US government to get out of the war. The British government would have faced much less public pressure to end the war than the US did.
Second, the British should have had fewer worries about the colonies getting help from rival superpowers. The French did, of course, want to help the colonies rebel, but they were in a much worse position to help than the Soviets and the Chinese were. There was no way the US was ever going to prevent the Soviets and Chinese from getting supplies to Vietnam. By contrast, the British should have been able to prevent the French from helping the colonists given the naval superiority they enjoyed.
These are, to me, the two major advantages the British had at the time of the Revolution when compared to the US in Vietnam.