Dec & Jan is winter in Northern Vietnam included Halong Bay, the highest temperature reaches to only 22oC while the temperature can drop to about 15oC. The sea temperature continues to lower to 23oC. The dry season remains with only 20mm and the number of wet days is only 7 days. There is only 5hrs of sunshine during the 11hrs of daylight in December. Visibility during December is quite low so photo taking might be disappointing.
Unlike Northern Vietnam, Hoi An is a city of two seasons with warm weather averaging 29°C dominating for most of the year. The wet monsoon season spans the months from September through to January with a dry season for the rest of the year.