The National Centre for Hydro-Meteorological Forecasting warned that saturated ground conditions could trigger flash floods and landslides in several northern mountainous localities.
In central Vietnam, hot weather is forecast on July 18-19, with temperatures ranging from 35 degrees Celcius to 37 degrees Celcius and exceeding 37 degrees Celcius in some areas.
The heat will affect provinces from Nghe An to Danang, as well as eastern parts of Quang Ngai and Dak Lak.
Forecasters said the heatwave is likely to persist until around July 23 before gradually easing from July 24.
Meanwhile, the Central Highlands and southern Vietnam are expected to see scattered showers and thunderstorms, with locally heavy rain in the afternoon and evening.
Authorities have advised residents in flood-prone areas to remain alert for flash floods, landslides and rising river levels, while those in areas affected by the heat are urged to take precautions against prolonged exposure to high temperatures.