Stay off the 1A, it's an ugly shitshow, take the Ho Chi Minh highway, make sure you go across the Hai Van Pass, and seriously, seriously, be careful. Watch out for buffalo coming out of the ditches onto the road.
Saw several people (experienced westerners and locals) die while doing this. It's not a joke and not a theme park, some people tend to treat it like that, though I am sure you won't. If you get hurt and go to the hospital, your insurance is not going to cover you.
Not trying to be a buzzkill, but I saw people some seriously stupid shit and pay the price, and also some very experienced people get very hurt.
Be safe. Don't know how much longer this will be doable, and it's an exhilarating experience.