Canadian-born Cathy Huynh and her friend Karin Bowerman, a 27-year-old American, were staying in a guest house in the coastal town of Nha Trang last week when Ms Bowerman suddenly became seriously ill.
According to reports from local news organizations, Ms Huynh took her to a local hospital where she was hospitalized with acute respiratory failure. She died later that night.
Two days later, Ms Huynh appeared to be suffering similar symptoms, with limited blood and oxygen flow to her internal organs, the Tuoi Tre newspaper reported.
She also died, prompting an investigation into the mystery deaths.
The Globe and Mail said several reports suggested the pair had been poisoned.
The incident follows the recent deaths of two Canadian sisters in Thailand. The bodies of Audrey and Noemi Belanger, aged 20 and 26, were found on June 15 by hotel staff on Phi Phi island, 800 km south of Bangkok.
The Toronto Sun said forensic officials found vomit in the room, blood on their lips and gums and their fingernails and toenails were blue. Lieutenant colonel Rat Somboon of Krabi Provincial Police added there were 'signs of serious food poisoning.'
Miss Huynh and Ms Bowerman were teaching English in Vietnam and had taken a short holiday when the tragedy occurred.
The family of Miss Huynh has set up a fund on a website to raise money to fly her body home to Canada for burial.