That's correct it's a calque of those mythologies. If you compare the Vietnamese names to the Chinese, they're almost exactly the same except for Pluto where Chinese uses 冥 which is "Minh" in Sino-Vietnamese and means deep or dark. The thing is, "Minh" is most often associated with the word 明 which is a homophone meaning "bright", quite the contrast! "Minh" is a contranym in Vietnamese and this is why they replaced that with 閻 (Diêm).