looking for a long lost book very similar to John Carter of Mars. Details are a bit hazy but the story is as follows. Man/soldier finds ancient ruins in i think Vietnamese jungle when hiding from enemy troops. In those ruins he falls into nephrite jade well which transports him to Mars where he discovers planet full of live and medieval civilization. Dont remember the rest of story but he learns fencing with sword and rides giant birds.
Sounds like it could be "Lieut. Gullivar Jones: His Vacation" (1905) by Edwin Lester Arnold. As Wikipedia explains, the work predates Edgar Rice Burroughs and might have likely been the inspiration for John Carter on Barsoom. The similarities are striking, though Lt. Jones is (in the novel) not nearly the heroic swashbuckler that was Cpt. Carter
The Question's reference to Vietnam suggests you might have been reading the Marvel Comic adaptation from 1972. I remember the comic well, where they "butched up" Gullivar considerably.
Marvel's version modernized the setting, recasting Gullivar as a Vietnam War veteran. Though this official adaptation used many of Arnold's characters and concepts, it was not a strict adaptation of the original book.