The low-pressure system was located in the area east of the southern Philippines on Sunday night, packing winds of up to 60km/h and squalls of up to 102km/h, according to the National Center for Hydro-Meteorological Forecasting.
The depression is expected to travel southwest at about 10 to 15km per hour, the center added.
Director Hoang Duc Cuong warned that the system would enter the East Vietnam Sea on Tuesday and gather strength to become a storm.
Under the influence of an enhanced cold front and storm circulation, it is forecast to rain in northern Vietnam, especially in the northwest region.
The weather conditions in the southern part of the East Vietnam Sea will also worsen in the next few days, Cuong added, before saying that the development of the tropical depression needs to be closely monitored.
A tropical depression or typhoon in the maritime area at this time of the year is not unprecedented, Cuong explained, listing the examples of storm Melor, which formed in late December 2015, or Sonamu, striking the same region in early January 2013.
However, these storms often weaken quickly during their journeys at sea due to the effect of cold fronts coming from the north.