There was a hindu civilization in the middle of Vietnam called the Kingdom of Champa. Rather advanced and quite spectacular cities, pre-bombing anyway. It seems to have been pretty dominant up to the mid 19th century. Look for books on that.
The Buddhist and Christian influences came from Western European and Chinese incursions, of course, and are not so interesting, though I think Champa was from an Indian incursion, too. The country itself, in fact, seems to be somewhat of a French colonial construct. Prior to that I think it was just a set of adjacent, but mostly unrelated regions, with different tribal populations. Hmong, Thai, Lao and lots more tribes. Champa may have been the major kingdom. The Emperors of Vietnam, even, were just French puppets, and the position was totally created by France.
But I am far from an expert, most of this information coming from guide books and an extended visit a couple of years ago. Great place to visit, including wonderful and healthy food. The further north one goes in the country, the better, too, because in the south the war still dominates things to a large (and sad) degree.
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Feeling (south) loopy