The Asia Pacific Gateway (APG) was damaged on Tuesday afternoon, a local Internet service provider told VnExpress.
A spokesperson of the provider, who asked that the company not be named, said a break has occurred in the cable's S3 branch 427 km from China.
On Tuesday night Internet users in Vietnam reported difficulties in accessing foreign websites like Facebook and Netflix.
Huynh Dung of Ho Chi Minh City said on Tuesday night he could not use Twitter, Google or YouTube as smoothly as usual.
Some people also reported connection losses, high latency when playing online games and significantly reduced download speed when using Google Drive.
All Internet service providers claim to have taken measures to handle the issue including by routing international traffic to other cables.
It is not clear how long it will take for the cable to be fixed.
APG, installed in 2016, is used by telecom service providers VNPT, Viettel, FPT Telecom, and CMC Telecom.
Last year it was disrupted four times, with the final one in December taking two months to be fixed.
This year there have been two disruptions, with the first in April lasting 10 days.
Vietnam has six submarine cables and an overland system that runs through China.
There are 69 million Internet users in the country of 96 million people, according to German data portal Statista.