The communist nation approved plans to build the plants in 2009 in Ninh Thuan province with an eye towards easing energy shortages brought about by its rapidly industrialising economy.
They were slated to have a capacity of 4,000 megawatts, developed with assistance from Russia's Rosatom and a Japanese consortium, and would have been the first nuclear plants in Southeast Asia.
But state-run media reported that the government has asked Vietnam's rubber stamp parliament, the National Assembly, to suspend the projects.
"The total investment has risen too high," Le Hong Tinh, deputy head of Vietnam's Commission of Science, Technology and Environment, was quoted as saying by the Tien Phong newspaper.
He said the proposed cost for both plants had doubled since 2009 to an estimated $18 billion as the government sought more advanced technology following Japan's deadly Fukushima nuclear disaster in 2011.
Tinh added that the projects could also pose an environmental threat and said the nation could not afford to risk another disaster after a toxic industrial leak triggered mass fish deaths earlier this year.
The leak ravaged local fishing economies and was pinned on a steel plant in the area run by the Taiwanese conglomerate Formosa.
"After the Formosa accident, we are paying more attention to risk and safety factors... we need to be more prudent. It is time and necessary for us to stop," he said.
The plants' proximity to island chains in the disputed South China Sea, most of which is claimed by Beijing, was an additional concern, he said.
Vietnam has said it will buy power from neighbouring countries and is also looking to boost its own energy production.
The country of 93 million relies mostly on coal and hydropower but has said it wants to increase renewable energy production in the next 15 years.
This week it announced a $2.2-billion deal with Ireland's Mainstream Renewable Power to build three wind farms in the country.