The acquisition came more than two years after the giant bought a stake of 70% in the US company for US$7 million.
Last year Vinamilk also raised its stake in New Zealand's dairy firm Miraka Limited to 22.8% from 19.3%.
The state-controlled company, which exports baby formula, soymilk and UHT milk products to 42 countries, currently runs a wholly-owned subsidiary in Poland and a factory in Cambodia with local partner Angkor Dairy Products.
The most recognizable brand in Vietnam with a brand value of $1.13 billion, Vinamilk is now one of the country's biggest magnets for foreign investors, after being allowed to remove a 49% cap for foreign ownership.
The approval was announced last week, as the government plans to sell its 45% stake in the US$7.85-billion company .
Vinamilk reported revenue of VND40.22 trillion (US$1.8 billion) last year, up 14 percent from 2014, and net profit of VND7.77 trillion (US$348 million), up 28%. It expects a 11% rise in revenue and 6% in profit this year.