I’ve not been to Cambodia but here are my Vietnam tips:
I’d recommend going to Cat Ba Island (instead of Ha Long)and spending a few days on a junk with these guys:
http://www.asiaoutdoors.com.vn
The scenery is very similar to Ha Long but much quieter. We climbed and kayaked during the day and spent the night of the junk with them and it was bloody brilliant. Bonus if your into climbing at all as you can do some DWS. The crew on the ship were locals and came swimming with us in the phosphorescent plancton after a few too many rice wines haha.
War based stuff: Doing the tunnels from HCMC was pretty cool, I really enjoyed that; our guide really opened my eyes to the Vietnamese perspective on the war (something that was confirmed by the recent BBC documentary). Also the War Museum in HCMC was excellent. The Hanoi Hilton was less good.
I really enjoyed the madness of Saigon but it’s a bit of a culture shock coming into it straight from the Uk due to the 40C heat, 100% humidity and 8 million people. Certain streets are very “gap yeaaaar” and others are very much not so.
We started there for 4 days so everywhere else felt pretty “quiet” in comparison. The jet lag was pretty wild and we slept for 24 hours on arrival.
Hoi An was lovely, very peacful and an excellent place to properly relax for a few days after Saigon (and buy a hilarious tailored suit).
I’d also add that Danang is pretty grim and I wouldn’t bother. It’s basically an artificial city which the government has forced as the third city of the country and thus it has very little history / interesting sites.
Out itinary was London > Saigon > Da Nang, taxi to Hoi An > Fly to Hannoi > London. Never got to visit the north which was my one regret from the trip, Sapa looks like a cracking bit of the country and i’m a mountain man at heart.