Wiktionary says đúng is a non-Sino-Vietnamese reading of 中 (used here to mean "correct") - standard reading would be trúng
Vietnamese is my first language and 'right' has always been 'đúng'. I always understood the word 'trúng' to be used more like 'I won something' or 'I got something'. I believe the Vietnamese reading for 中 is 'trung' as in 'Trung Quốc' aka China.
“Non-sino-Vietnamese” is tricky since it could include words that were borrowed from Chinese much earlier, but have not been proven to be of Chinese origin. Because of phonetic similarity, đúng is plausibly such a word.
Hmm, some northern Chinese still use this slang, when you agree with something or someone would say: 中 instead of 對.