@ridethecliche That's correct, hopping flights from Hanoi to Hue, Danang to Dalat, and Saigon to Siem Reap to save time and avoid riding buses whenever possible. Thanks for the advice about the flights, I'll be sure there's nothing mission-critical around my flight-times so I don't get screwed by delays.
To @TomOwen's point above, I will be taking the train from Hue to Danang, I really look forward to it. Will be spending a night in Hue and there's some flexibility in my schedule for Nha Trang or Danang but not both. I'll see what I can do, but I have a feeling that the buses from Dalat to Mui Ne and from Mui Ne to Saigon are gonna take up a really long time and be draining (no other way to make those journeys as far as I can tell... there may actually be a train for the latter but there's some work being done near Saigon and trains seem to be stopping short of it). I was thinking of heading to Halong the morning I get back from Sapa for a 2d/1n excursion. Would you recommend otherwise? Cat ba would be great, should I be able to find a ferry there from the same part of Halong Bay where the bus drops me off?
edit: apologies to anybody who prefers me to use HCMC instead, I understand it's a sensitive topic. I'm just being lazy, I guess.