Rainfall of 60-120mm is forecast across most areas from the evening of May 29, with isolated totals exceeding 250mm and intense downpours of more than 100mm within three hours.
The weather agency said heavy rain could trigger flooding in low-lying urban and industrial areas, as well as flash floods and landslides in mountainous regions.
Elsewhere, scattered showers and thunderstorms are forecast for northern Vietnam and the north-central region on the evening of May 29, with rainfall of 15-30mm and isolated areas receiving more than 100mm.
Eastern areas from Gia Lai to Khanh Hoa and parts of the Central Highlands are also forecast to see scattered showers and thunderstorms, with rainfall of 10-30mm and isolated totals above 60mm
Forecasters warned that thunderstorms could be accompanied by lightning, hail and strong winds.
The agency said rainfall across southern Vietnam and Lam Dong is expected to ease from the night of May 31.
In Cao Bang Province, rainfall of between 10mm and 30mm, with some areas receiving more than 50mm, was forecast between late afternoon and evening on May 29.
Authorities warned of flash flood and landslide risks in several localities, including Quang Trung, Can Yen, Dinh Phong, Ha Quang, Quang Han, Thanh Long and Thong Nong.
Heavy rain earlier in the day brought rainfall totals of 86.6mm in Tri Phuong and 50.8mm in Thong Nong, while soil moisture levels in some areas were already approaching or reaching saturation, according to meteorological data.
At sea, showers and thunderstorms were forecast for parts of the East Sea, including the Truong Sa (Spratly) archipelago, the Gulf of Tonkin and waters off southern Vietnam.
Forecasters warned that thunderstorms could generate waterspouts, strong winds exceeding force 6 and waves higher than two metres, posing risks to vessels operating in affected areas.
