In the 45 kg category, Hieu, 20, beat Filipina Rudzna Abubakar 29-28 in the final on Saturday with a knock out, despite being smaller than her opponent.
She chose a quick pressing strategy and used her elbows and knees as the main weapons to win her first ever world championship gold.
In the 51 kg category, Ly, 24, defeated Ekaterina Gurina 30-27 on Sunday. Her Russian opponent attacked relentlessly but Ly defended solidly and responded with accurate counters with her right foot.
She also used clinches to lock her opponent, preventing her from throwing punches. She won all three rounds on points.
Ly is Vietnam’s biggest hope in muay Thai in international competitions. She has won four golds in world championships, one in the SEA Games, one in ASEAN Indoor Games, and two in ASEAN Beach Games.
The 2019 World Muay Thai Championship took place in Bangkok from July 18-28, and attracted top fighters.
It consisted of 15 different weight classes in four categories, boys, girls, men and women.
The event served as a qualifier for the 2021 World Games in the U.S., with each men's and women's team that made the quarterfinals earning points for the qualification.
Vietnam sent 17 fighters. While the women won two golds, the men failed to even reach a final.