I looked up the two without Sao and found they were Marine Kingdom or King for Neptune and Hell King for Pluto. Are these the names of the Roman gods in Vietnamese added to Sao?
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).