January is understandably one of the peak touring months as the weather is generally good throughout, but Vietnam’s geography means you will experience a range of temperatures if you travel from north to south.
Most start in the north and this makes sense, as although it should be dry the average temperature in Hanoi will be around 18°C, so an extra layer will probably be needed in morning and evening, especially if overnighting on a Halong Bay cruise. The high northern hills in Sapa and Ha Giang can be visited will be cool and can be foggy.
Moving down to the central coast (Hue & Hoi An), the wet season is starting to draw to a close, especially in the 2nd half of the month when average temperatures should be approaching the mid-20s°C. Not beach weather, but ideal for touring.
The story keeps on improving as you head south to Saigon, Mekong Delta and the beaches of Phan Thiet, Mui Ne, Phu Quoc and Con Dao. The weather should be perfect, a balmy average of 28°C, low humidity and wall to wall sunshine.