Sung Seo Trang, 34, and Giang Seo Tinh, 33, were stopped on their way to sell the pills at a motel named Ngoc Tuan in Bac Lenh Ward, police said.
The two suspects said Trang received the shipment from a man named Khai, who has yet to be identified, about six days ago.
Trang then called Tinh and told him to hide the package while he found a buyer. On the afternoon of March 31, he called Tinh and told him to bring the pills to the Ngoc Tuan Motel, but the deal fell through.
The following day, police raided the motel and caught Tinh and Trang trying to sell the pills again.
A subsequent search turned up two packages wrapped in silver foil containing a total of 4,000 pills. The suspects admitted they were synthetic drugs.
Vietnam has some of the world’s toughest drug laws. Those convicted of smuggling more than 100 grams of heroin, cocaine, methamphetamine or ecstasy are punishable by death by lethal injection or 20 years in jail.
Although the laws are strictly enforced with capital punishment handed down regularly, drug running continues.