Nhi defeated Kanyarat Yoohanngoh of Thailand in 10 rounds to take the World Boxing Organization (WBO) Asia Pacific Minimumweight crown in Cambodia.
The 24-year-old went into the bout the underdog with just three wins in her professional career compared to Yoohanngoh's seven.
But her confidence and relentless attacks forced her opponent to resort to illegal moves, which resulted in a point deduction by the referee.
"I am happy with the victory but was disappointed the match went 10 rounds."
Nhi, from the southern An Giang Province, is a rising star in Vietnamese boxing.
In 2018 she had defeated three-time world champion Filipino Gretchen Abaniel at the Victory 8 championship.
She will be the first Vietnamese to enter the WBO World Championship, which this time will be held in South Korea in April.