A'i nam a'i nu+~" (or "a' nam a' nu+~") is probably the closest descriptive approximation to what is meant ontologically (and perhaps even behavorially if one takes "a'i" as a verb -- but then the total meaning of the term is rather misleading/misguided since it'll be closer to what's understood in English as "bisexual" behavior). The earliest usage of this term that I can find so far is the 1940s. Another variation on this term, which is more common in biological/medical vocabulary, is "ba'n nam ba'n nu+~", which, like the others, means literally "part/half man and part/half woman".

Bo'ng la.i ca'i" is obviously an underground slang. I suspect that it's associated with cult practices of "ddo^`ng bo'ng". There are many conflicting semantic and cultural exegeses on the various (high-charged) terms: "bo'ng", "la.i" and "ca'i" (as well as "ddo^`ng bo'ng" and "ddo^`ng co^ bo'ng ca^.u", "con ba` co^", though probably NOT "co^ ho^`n ca'c dda(?ng" as usually understood, etc.) This term, if it's associated at all with quasi-shamanistic/spirit-cult practices ("ddo^`ng bo'ng", "vi'a co^'t" etc.) then its flavor is distinctly more ontological, even though it can also have a behavorial dimension in the figurative sense that is used for homosexuals/hermaphrodites and other sexually ambiguous/queer categories of people. I am unable to date this term (and its host of derivatives/correlates), even though it surely predates the 1960s. "Bo'ng" and "la.i ca'i" are the most common abbreviated correlates of this term (even though "la.i ca'i" very often stands on its own with regards to gay male, given its literal meaning of "contaminated with the feminine" -- but then can be used as a byword for "effeminacy" which isn't exclusive or even characteristic of gay males at all, as we all know! :-))

Pe^ dde^" ("pe'de'", short for the French "pe'deraste" whose English counterpart is the Latinate cognate "pederast") is probably the most common modern/contemporary (post 1950s/60s?) byword for a gay person in Vietnam. It's probably urban in origin/scope and can probably be dated to the French usage of the word (albeit much less commonly than the pervasive "inverti" which doesn't seem to have a literal correlate in Vietnamese: its English correlate is the less commonly used "invert").

Ke^ gian" is a much more ancient term, borrowed from Chinese (and dating back quite a few centuries), which is mostly used to depict the anal kind of "sodomy". It's a description of the act, and not so much a person. Needless to say, it was included in earlier Sino- Vietnamese dictionaries (the earliest instance of which I have been able to date to 1898). But then, as is made clear in both the Chinese and Vietnamese context, "ke^ gian" can often mean a generic term for the tabooed kind of "anal intercourse" and not limited to same-sex practice. (And in medieval Europe the term had been used broadly to encompass all sexual deviancy, including bestiality, so there!).