It is a bit cold in the end of January in Halong Bay. The temperature is around (max) 16-18 °C. There might be drizzles (as it will be Spring in north Vietnam). The weather will not allow you to enjoy swimming or kayaking in the bay. But there are still many things to discover in Halong (Grotto, Cave...)

As it will be Vietnamese lunar new year eve (aka Tet), things will be a little bit more expensive than usual. During Tet (about 4 to 5 days), it is event not easy to find the services.
There are lot of things to do in Halong Bay.
Halong bay cruise tour is also a good choice for foreign travelers and domestic travelers also. So far, it is the best way for traveler to discover and experience the whole Halong bay.

Wish you have a nice trip to Vietnam