December is early in Northern Vietnam winter season. The Norhern and central regions of Vietnam have little rains and mostly cool weather.
Hanoi and Halong will be cool and dry. The temperature may be down to +6 in late December, it is also wet and rainy. Snow in Sapa, with icicles in the water bottle in the morning.
Hoi An and Hue might be monsoonal.Danang to Nha Trang can get down to +20'C in december, rains stop in mid December. Ho Chi Minh City will be warm and dry. In general, December will be a perfect month to travel.
Reservations:
December is among popular travel months in Vietnam, advanced reservations are strongly advised.
Costs
Prices reflect the popularity of travel at this time plus the Christmas holidays and New Year Festival.
The city weather channel
Hanoi weather
Ho Chi Minh City weather
Sapa weather
Halong bay weather
Hoi an weather
Hue weather
Nha Trang weather
Dalat weather