The project management board recommended the four-carriage trains for the USD891m project be supplied by Beijing Rolling Stock Equipment Co.
Thang told reporters he had received many messages advising him not to buy the trains, some suggesting a private relationship with the Chinese supplier.
"I have received threats if the ministry buys the trains," Thang added.
Thang said the purchase was based on a 2008 agreement between Vietnam and China, which is the major supervisor, contractor and investor for the project.
"I recognise the weakness of the contractors of the project and I have wanted to replace them many times, but there is a contract," Thang said.
Thang said Vietnam had entered many overseas direct investment contracts with China, Japan and South Korea.
The Cat Linh-Ha Dong urban railway project is for 13km of rail and was for initial capital investment of USD552m. Construction began in 2009, but is running behind schedule and cost overruns have raised the investment to USD891m.
The project was scheduled for the completion by late 2015, is now expected to be completed next year.
The Vietnam Railway Authority is the project developer, while China Railway Sixth Group, a subsidiary of construction conglomerate China Railway Group, is the engineering, procurement, and construction contractor.
In November 2014, construction of Cat Linh-Ha Dong railway project was suspended for several weeks after a passer-by was killed and two others injured when a girder fell from a crane.
On May 12 this year, a half-tonne girder fell from a crane, crushing a Honda Civic. No one was injured.
Dtinews