The newspapers hit reportedly included the popular Thanh Nien, Tien Phong, Sai Gon Giai Phong, Voice of Vietnam, Zing, and Bao Moi, all of which use the ePi content management system hosted on VinaData, a data center owned by tech company VNG.
According to an email VinaData sent to its clients, the disruption – which began at 9 a.m. -- was due to a power cut at Quang Trung Software City (QTSC) in HCMC, where the data center is located.
In addition to the news sites, VNG services such as music player application Zing MP3, messaging application Zalo, online television service Zing TV, payment gateway ZaloPay, and VNG's online games also became inaccessible.
After power supply resumed, the online newspapers and Zing's services came back on at around 4:30 p.m. But Zalo, ZaloPay and VNG's online games remained inaccessible as of 11 p.m. Sunday.
VNG's services are widely used in business and communication.
Speaking to the media Sunday afternoon, Nguyen Tan Hung, public relations chief of the Ho Chi Minh City Power Corporation, said the fault lay with VNG since it was a scheduled power cut and QTSC had been informed five days earlier.
Hung claimed the blackout occurred due to an issue with VNG's backup power generator.
But a tech firm spokesperson refused to comment on this.
According to security expert Nguyen Hong Phuc, it is "unacceptable" for such issues to occur at a data center just because of a power cut.
"Electricity, network and cooling system are the three bases of a data center and the reason customers use its service instead of hosting their servers at home.
"The time it takes to carry out maintenance or resolve incidents such as power cuts shows the capability and quality of a center. If the downtime is one minute then it's normal, five minutes is a big issue and 30 minutes to one hour is unacceptable."
Many users of VNG services have expressed frustration.
"I could not contact my customers on Zalo, I missed a lot of work," said Bich Huyen, who works for an insurance company in Hanoi. Zalo is a top messaging app in Vietnam and Huyen said it has been her main contact tool.
Duc Huy, who was paying his electricity bill via ZaloPay when the service was disrupted on Sunday, was worried that his payment had not gone through. "I'm not sure if I would get a refund," said Huy from Ho Chi Minh City.
The incident Sunday is the second time this year that VNG has been hit by a systemic problem. Last April detailed personal information of over 163 million Zing ID account owners were leaked on an online forum.