In a trial on Friday, the Chau Thanh District People’s Court in Hau Giang Province sentenced Nguyen Thi Be of Can Tho City to nine years in jail.
She had been sentenced to eight years in jail the previous week on another robbery conviction.
Be said she befriended older men on the street and they would call her soon after to meet at hotels where she would spike their drinks
At the Friday trial, the court heard that Be lured a 83-year-old man in Soc Trang to a hotel last year.
She had told the old man that she wanted him to be virile and told him to drink some medicine, which was actually a drug.
After he became unconscious, she robbed him of VND500,000 (US$24) and two cell phones.
The court also found Be guilty of convicted for stealing a gold ring from a 43-year-old man last year using the same trick.
Be, who was Can Tho deputy director of a security firm in Soc Trang, said she stole money because her husband had died and she had to raise two children and look after her old mother.
She had borrowed money at high interest and was unable to repay the loans, she said.
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Be is being investigated for 20 similar cases that happened between 2009 and last October, when she was caught.
She said she found potential victims at public places like bus stations, markets, hospitals and industrial parks in Can Tho and Hau Giang.
She also said she'd had sex with some of of her victims before drugging and robbing them.
Among the cases, Be is accused of killing a 59-year-old victim in August 2011 with tranquilizer overdose.
Be has confessed to having stolen cash and cell phones worth approximately VND100 million (US$4,770).
Police said Be was using a strong tranquilizer, which could render someone unconscious in five minutes and could also prove fatal.