In terms of buildings, culture, and history. I would prefer it be narrowed down to a small area and not just a city or something if possible. I am sure there are pockets of rich history all around the country.
So a specific location that is very niche?
One of my favorite places to see, having gone twice now, is the Vinh Moc Tunnels. They are amazing to see in person. You can read about them online. There is a very small charge to enter but it is worth it and helps support the locals.
If you like history and are a history buff, you may enjoy a trip to "Hell's Hole" aka Hoa Lo Prison. It is a museum dedicated to the area. It has four different areas if I am not mistaken.
Ahh they is so many places! Do you have any interests in particular? Do you want religious history? Maybe a bit of the art scene? Farming culture?
In terms of specific locations, I would recommend checking out Ke Bang National Park. The caves are stunning and have a lot of history with them. You can take a guided tour too.