Downpours plus strong winds uprooted many trees in Quang Ninh Province's Mong Cai Town.
People have been recommended not to go out. Vessels have been brought to safety and people in landslides-prone areas evacuated, officials said.
The town that is home to more than 80,000 suffered blackouts from 9 p.m.
Many parts of Quang Ninh, home to world-famous Ha Long Bay, have received rainfall between 115-240 mm from Thursday afternoon.
Heavy rains also poured in the nearby Hai Phong, flooding several streets. Coastal entertainment activities in Hai Phong have been suspended.
The National Hydro-meteorological Forecasting Center said two hours after sweeping through Quang Ninh, Maon weakened into a tropical depression with wind speeds of 39-61 kph over Vietnam's northern highlands.
Maon is the third storm to make an appearance on the East Sea this year. Due to the La Nina phenomenon, storms would come later this year, mainly during the latter half of the year and may come even in January next year.