The decision was made Friday by the Athletics Integrity Unit (AIU), which oversees fights doping in track and field.
Lan had finished second in 55.30 seconds in the race on August 27 in Indonesia last year. Kemi, of Bahrain, had timed 54.59 seconds.
Her gold takes Vietnam’s tally in the quadrennial event to five.
The AIU said Kemi, 26, tested positive for the steroid stanozolol.
Stanozolol, which boosts muscle growth and protein metabolism, is on the banned list of the World Anti-Doping Agency.
She will be banned for four years backdated from November 26, 2018, when her violation was detected.
Bahrain loses the gold medal in the 4x400 mixed relay to India.
Lan, 23, is one of Vietnam’s top runners.
She won the 400 meters at the 2019 Asian Grand Prix Series in China in June and the 400 meters hurdles at the 2019 Asian Athletics Championships in Doha, Qatar, in April.