My grandma and dad took a look at the Duolingo course for Vietnamese and immediately derided it as told me about the "communist-taught version" of Vietnamese, with a lot of words that were changed around for a vague reason of establishing power. I highly doubt it, but i'd like to get a second opinion.
Where in Vietnam did your grandparents come from? The Vietnamese language has a large amount of dialectal variance compared to English, and while northern Vietnamese (roughly around Hanoi and the provinces of Thanh Hoa, and Hai Phong) and Southern Vietnamese ( spoken from Quang Nam downwards) tend to dominate education and public TV and radio, northern Vietnamese tends to be used as a standard (which was were the communist power base is)
Now, if your grandparents are from centre Vietnam, their dialect is very different; the provinces of Quang Binh, Quang Tri, and Thua Thein-Hue are remote from the rest of Vietnam (due to the Hai Van and Ngang passes) and have developed a very different (and a much more conservative) version of Vietnamese, which hasn't gotten much prestige compared to northern and southern Vietnamese
But I'd ask ask linguistics for a more in depth idea of the linguistic differences in Vietnamese
I'm not sure if "bastardization" would be the best term, but there was, and still is, a movement to change Vietnamese to a northern "standard." This is especially easily spotted in written language.
For example, pre-1975, hospital was spelled "bịnh viện," as it is pronounced in the south (you can Google image search for some old pics). However, it is now spelled "bệnh viện," as the "standard" northern way.
Thomas Bass wrote a series of articles after struggling to get the Vietnamese translation of his book published in Vietnam, and has an interesting discussion of how "editors" based in Hanoi fought against him using the term "swamp of assassins" (rừng sát), instead changing it to "swamp of tidal shrubs" (rừng sác).
I know that anecdotes are not allowed here, but if you want, I have many I could PM you about many similar things that occur in southern Vietnam regularly, even today.
Also, what words/terms from Duolingo do your grandparents say are the "communist-taught" Vietnamese? I'd be curious to know. PM me if you wish.
Changing words to establish power ? I haven't heard anything about it. Would you please give some specific examples (in Vietnamese)?