Be careful of pickpockets. They're pretty skilled over there, and tend to work in teams.

I did my three week trip in the opposite direction (south to north) some years ago. The backpacker tours were so cheap then (about $30/day with lodging and meals) that you couldn't save much money going independently. My biggest two were 3 days in the Mekong Delta (overnights in Cantho and Chau Doc) and 2 in Ha Long Bay. Buses were charters, but one broke down and the guide put us on the public bus. A traffic cop saw that there were foreigners, stopped the bus and the guide had to pay a "fine." It was a bit cramped and there were chickens and other livestock.

Intercity transit was always slow, even if on time. Don't apply western thinking to travel times and distances.

I had my food bout around Hoi An, which slowed me down for a day or two. Still made it out to My Son.

Also lots of cheap daytours out of HCMC and Hanoi that were worth it, like Perfume Pagoda and that river, and the Caodai holy see and the Cu Chi tunnels.

I normally don't do the tour thing that heavily, but it was so cheap with Vietnam and it removed hassles so much that it was hard to resist. You'd also see the same bunch of people along the way. I was pretty conventional with HCMC for a few days, the Mekong Delta tour, Da Lat for a day or two, Nha Trang (back in the days of the legendary Mama Hanh boat tours), Hoi An, Hue, Hanoi, and Ha Long Bay.