They found Nguyen Thi Thai Dung, 23, seated in a car parked in front of a hotel on To Hien Thanh Street and on checking the vehicle found 15 bottles of the concoction.
Dung told them she bought the marijuana herself and added it to the tea to sell mainly to tourists.
"I only make a little bit... I don't make too much like other people," she reportedly told them. She allegedly sold up to 20 bottles a day.
The police have arrested her.