Players had to create accounts and buy virtual currency with real money to use in the gambling.
The network had been in operation for a long time and attracted thousands of gamblers and stakes of around VND3 trillion ($128.92 million) this year alone, investigators said.
Vietnam relaxed its stance on gambling as a "social evil," legalizing sports betting in 2017 and allowing people aged over 21 with a monthly income of at least VND10 million ($432) to gamble in a casino on the southern Phu Quoc Island.
But online gambling remains illegal.
Illegal gambling and organizing it carry fines of VND10-50 million ($430- 2,150) and jail terms of three to seven years.