The 24-year-old ran in a time of 56.10 seconds, her fastest this year.
She beat 400 meters hurdles specialists Bahraini Aminat Yusuf Jamal, the silver medalist at the 2018 Asian Games, and India’s Sarita Gayakwad, who had won gold.
Lan got off to a great start and kept the lead until the end of the race. Jamal finished second in 56.39 seconds and Gayakwad third in 57.22 seconds.
However, Lan was 0.10 seconds slower than the qualifying mark for the 2020 Tokyo Olympics.
The 2019 Asian Athletics Championships are taking place in Doha, Qatar, from April 21 to 24 with athletes from 63 countries.
There are 12 Vietnamese athletes at the event.
Lan will have an opportunity to win another gold when she and her teammates take part in the 4x400 meters women’s relay on Tuesday.
She won two silver medals in last year’s Asiad and a gold medal in the 2017 Asian Athletics Grand Prix in China.