The National Centre for Hydro-Meteorological Forecasting said the depression was over coastal waters off Quang Tri on Sunday afternoon, packing winds of 39-61 kilometres per hour and gusts of up to level 8-9 on Vietnam's 12-level scale.
The system was moving west at 10-15 kilometres per hour and was expected to make landfall between Ha Tinh and Quang Tri overnight or early on Monday.
Con Co special zone in Quang Tri recorded level 6-7 winds, gusting to level 8, while Dong Hoi recorded level 6 winds, gusting to level 8.
Coastal areas from Ha Tinh to Quang Tri are forecast to see level 6 winds, gusting to level 8, while inland areas could experience level 5-6 winds and gusts of level 7.
From Sunday evening through Tuesday, Thanh Hoa to Quang Tri is forecast to receive 150-350mm of rain, with some areas exceeding 500mm.
The northern midlands and delta and southern parts of Son La and Phu Tho are expected to receive 20-50mm overnight, with isolated totals above 100mm.
From Monday through Tuesday, rainfall of 70-150mm is forecast across the northern midlands and delta and southern parts of Son La, Phu Tho, Tuyen Quang and Lao Cai, with some areas receiving more than 200mm.
Heavy rain could cause flooding in urban, industrial and low-lying areas and increase the risk of flash floods and landslides in mountainous areas. Thunderstorms, whirlwinds and strong gusts are also possible.
Rivers from Thanh Hoa to Danang could see a flood wave through Wednesday. The Ngan Sau and Ngan Pho rivers may rise to alert levels 2-3, while other rivers are expected to reach levels 1-2, with some exceeding level 2.
A high to very high risk of flash floods and landslides has been flagged for parts of Ha Tinh, Quang Tri, Hue, Danang and Quang Ngai.