In December, it is often cold in Hanoi and especially in North-western region such as Sa Pa. Snow occasionally comes and leaves but it might get colder than one expect. Prepare warm clothes or you might be trapped in an undesirable flu. Also bring a lot of lip balms and hand-cream, for the dry weather may make you suffer intensely. As you move southward, it is not as dry and cold. There are pleasant breezes and the sun is on until late afternoon.
Hanoi weather in December
Ho Chi Minh City weather in December
What to expect if you travel to Vietnam in December
Flights to Vietnam and other exotic destinations may rocket closer to the Christmas time so plan your travel way ahead. If you are away from home during Christmas, take into consideration that mashed potatoes and roasted turkeys are not easy to find in Vietnam. On the other hand, you will have a festive holiday being abroad, trying bun cha and nem ran.
* December 22: Vietnam Military Day
* December 25: Christmas
* December 31: New Year's Eve
Read more: Best time to visit Vietnam