1 more reply for OP:
If you're going the backroad route, know how to drive on 2 wheels before you arrive. Renting a bike for a couple weeks (assuming you're leaving where you came in, likely HCMC) will save you a lot. Should be no more than $35 for 2 weeks, and gas is next to nothing. Just be sure to 'shop around' for a good bike in HCMC. There's lots of places to look. I can give a personal recommendation for one or two if you like. I'd suggest an automatic, last 3 years, Honda or Attila. The automatics are easier to drive and significantly more comfortable, plus heavier, which is safer. Note you can take bikes with you on trains and buses (you won't want to cross country on these things, trust me). Costs 1 extra seat fare usually. If you rent from each hotel you stay at, not only will you likely end up paying $10-15 per day, but you'll get crappier rentals too. Just make sure to keep 200K, 200K, 500K in your bike at all times ($40USD total, in those 3 bills though). Cops might impound them if you don't have bribes and get caught outside the province you rented from. This is rare but it happens. Probably also buy your helmets state-side. If you do buy in Vietnam, go to Andes in HCMC, it's on Nguyen Thi Minh Khai near the reunification palace.
Again, if you want specifics on places/restaurants/hotels/ideas, PM me.
Don't plan to see the south, center, and north in just 2 weeks. I would personally just go up to Hue and return for your flight home. You probably won't even make it that far, which is fine. Hue is a great little city, with decent historical sites to see if you like that sort of thing, but there's plenty to fill 2 weeks in elsewhere.