January is the coolest month throughout Vietnam. Hanoi and Halong may experience the coldest winter days with temperatures falling to a minimum of 10°C (50°F). The far north will be freezing for some times, even snow may be seen in Sapa and Lai Chau.
Hue and Hoian is quite cool and dry though the central region is still in late rainy season. You may enjoy your stroll around with a light jacket.
Nha Trang and Ho Chi Minh will be pleasant with temperature revolving around 27°C (81°F). Humidity is also lower than other months.
January is not the best month to travel in Vietnam but if you want to experience a Tet Holiday in Vietnam (usually from late Jan to early Feb), be sure to plan well ahead.
As the whole country is moving fast to prepare for Tet, prices usually peak up this time. This is general information for January, and an estimation for the year of 2011 and 2012, drawing from 2010 January observation.