The performance was worse than the national record, 56.15 seconds, she set at the 28th Southeast Asian Games in Singapore last year, where she won a gold medal in the same event.
Huyen, 23, also failed in the 400m event a few days ago when she finished her heat in sixth place in 52.97 seconds, ranking 43rd out of the 57 runners.
Her underperformance in both events was not surprising as she gained a ticket to the Olympics at the final moment and did not have time to gear up for the Games.
The women’s 400m hurdles is the only event Vietnam competed in on the 10th day of the Olympics.
With one gold and one silver medal from marksman Hoang Xuan Vinh, Vietnam is currently in 37th place in the medal standings.
The US remains in top position with 75 medals, including 26 gold, followed by the UK and China.