The region has been experiencing light rains in the morning and night due to moisture-laden winds blowing in from the sea.
Temperatures in Hanoi have been going up to 19 degrees as a result, while highland areas like Sa Pa in Lao Cai experiencing temperatures of up to 15 degrees.
They are expected to rise to 22 degrees by Friday noon before plummeting due to the cold winds, according to the National Center for Hydro-Meteorological Forecasting.
It forecast mountainous areas to get snow and frost.
A low-pressure area moving in from Laos would also bring rains, with some regions getting up to 80 mm within 24 hours.
Meteorologists said the mass of cold air would be the strongest this winter.
U.S. weather forecast service AccuWeather said Hanoi would see temperatures of 18-22 degrees on Friday before they drop to 9-12 degrees and then to 8-10 degrees on Sunday. They would then slowly rise to 16-20 degrees over the next few days.
Central Vietnam will also feel the impact of the cold mass starting Sunday, with Thanh Hoa experiencing temperatures of 9-11 degrees. In Da Nang, they will drop to below 20 degrees at night this weekend.
There have been several cold spells since the end of December, with the coldest causing temperatures to plummet to 2 below zero on Mt Fansipan in Lao Cai.
In March and April average temperatures across the country would be around 0.5 degrees lower than usual, meteorologists said.