Sapa can be great, but iirc OP is going in winter, and it can be hit or miss. Fog/low clouds and rain can make it quite different than summer visits.
Vietnam in my mind can be broken into several parts for tourists. The north has more of the cultural history. Pagodas, ancient temples, influence from Chinese dynasties. It also has some nature in Sapa and the mountains, which are a full day further north, and Ha Long bay and Ninh Binh closer to Hanoi.
The central area has mostly nature mixed with a small amount of history. The huge caves go along with many jungle areas, and then there are the beaches along the coast. The history is the ancient capital of Hue and the somewhat touristy-but-lovely Hoi An and surrounds (riding bikes through the rice fields and towards the beach can be great if you're active). There is the conflict history for those few who might want that. There is also some Cham history in the central area, but not especially well preserved that I'm aware of. More like scattered ruins here and there. (The Cham people are not well known outside of Vietnam, but they were the enemies of, and eventually sacked, Angkor Wat and the people in the Siem Reap area, and they were some of the first Muslims in SE Asia as I am told.)
The south is all about food and delta life. There is culture if you like to see people and observe and talk with them, but there is not much in the way of buildings and monuments and temples etc. You can visit floating markets, eat delicious food in tiny alleys, and take boat tours through the jungle. There are a few beaches, but they are generally over-burdened and dirty, and not as scenic as some of the top beach destinations in the world. I would definitely not come to Vietnam for the beaches, but if you're nearby and you like beach vacations, they are ok. I would say for all of Vietnam's beaches, Hawaii and the Caribbean outshine almost always. Partly geography (the flow from the Mekong, the dropoffs to the South China Sea), and partly due to refuse. It's just not in the culture yet to take care of their environment.
Just to be clear, I'm not Vietnamese at all. I've lived here for a few years now, but I'm from the US. So I could easily have some of this wrong and it's just my opinions from an outsider point of view.