Nearly 3,900 houses, more than 22,000 hectares of crops submerged in massive flooding in southern Lam Dong province.
Matmo is likely to continue strengthening as it nears the southern coast of Vietnam, with winds gusting up to 90 kilometres per hour (56 miles/h).
The storm is expected to make landfall north of Nha Trang overnight Wednesday into Thursday and begin weakening as it moves inland.
The storm is not expected to make a significant impact in terms of wind damage, but the amount of rain will form the main risk.
Showers have already been making their way along the coast and Quy Nhon has already picked up 40mm of rain in the last 18 hours.
While the higher terrain near the coast of Vietnam will work to break down the system, the mountains will also cause the rain to funnel through the valleys, instigating flash floods and mudslides in the coming days.
Rain will be torrential at times, with 200-300mm of rainfall expected from Tuy Hoa north to Vinh in the next two days.
By the time the weekend is over, the remnants of the storm could bring those figures to over 350mm, particularly further inland.
A storm surge of two to three metres will be seen along the coast along with dangerous rip currents.