Ho Chi Minh City customs in June seized nearly 1.9 tons of plant-based drug khat disguised as tea or herbs in parcels from Africa.
Two delivery companies and more than 10 individuals are now suspected of deliberately smuggling the stimulant leaves.
The leaves were found in 129 packages from Kenya and Ethiopia either handled by the companies or with the individuals named as the recipients.
Khat is a shrub grown in East Africa and the Arabian Peninsula and its leaves are chewed for their stimulant effects, thanks to cathinone and cathine among the ingredients.
Another ton of the leaves were seized at Tan Son Nhat International Airport in the city earlier this year.