I don't know if this is a known issue, but importing text in Vietnamese to use with the reading tool does not seem to work. I tried importing the captions from a youtube video and the resulting text was all garbled, with few words recognized as words (the rest a mass of text without spaces). I thought it might be some issue with the importer so I imported a passage by copying the text (lyrics to a song) and the result is the same (so the importer worked). Also, the passages I create using Vietnamese as language seem to disappear from any list (there are no passages listed in "my passages" in Vietnamese).
I will try creating it again: https://oplingo.com/reading/12554/read
And I'm attaching a screenshot to show how it gets if the passage can't be accessed.
The original video is at MIN - ‘TRÊN TÌNH BẠN DƯỚI TÌNH YÊU’ OFFICIAL MUSIC VIDEO - YouTube