The National Centre for Hydro-Meteorological Forecasting said rainfall of 100-200 mm was expected across northeastern Vietnam and Thanh Hoa, with 200-300 mm in parts of the northeast and isolated areas exceeding 500 mm.
The heaviest rainfall is forecast in Lang Son, Quang Ninh, Haiphong and Bac Ninh, with rainfall exceeding 100 mm possible within three hours.
Rain is expected to ease from the night of July 5.
Elsewhere, Nghe An to Quang Tri are forecast to receive 15-30 mm of rain on Saturday, with isolated areas exceeding 100 mm, while the Central Highlands and southern Vietnam could see scattered showers of 10-30 mm, with some areas receiving more than 70 mm.
Thunderstorms may bring tornadoes, lightning, hail and strong winds.
The weather agency also warned of flooding on rivers in northeastern Vietnam between July 4 and July 7, with water levels expected to rise by 3-6 metres in upstream sections and 2-4 metres downstream.
It warned of flooding in low-lying urban and riverside areas across the Red River Delta, as well as flash floods and landslides in northern mountainous and midland provinces.