Some things I’ve noticed about Vietnamese vowels in the Saigon dialect:
‘u’ is like the pure vowel in European languages but can be closer to ‘o’. To me it sounds more like ‘u’ when the final sound is a stop t/c/p then when it’s an open vowel ending word so for example ngủ sounds closer to ngo. It can be hard to differentiate đủ and đổ.

I mentioned before about Vietnamese vowels that â and ơ are basically long and short versions of the same vowel as some linguists have said. According to Wikipedia they are /ɜ/ vs. /əː/ and I don’t think there is anything in English exactly like them. In the Hanoi dialect â is pronounced as /ɜ/ whereas in the Saigonese dialect it’s pronounced /ɨ/ or /ɜ/, the same as đến which I mentioned before. I was told that ơ was pronounced by spreading the lips outward laterally like a smile.
Many vowels have phonetic offglides in open syllables, meaning that they changed slightly into another vowel in the end. This happens with i and e but not a so “tri” will sounds sort of like “tri-y” and “tre” will sound like “tre-uh”.