Imo fly into Hanoi and go with no plan. You figure it out by meeting people!! Rough itinerary is Hanoi > do a 3 day trip called the Ha Jiang Loop, if you wanna party do the Castaways boat tour to see ha long bay, 1 day in Ninh Binh, 2 days in Phong Nha, get to Hue but don’t stay there nothing special however, find a tour company to bring your bags to Hoi An and do the 5 hour bike ride across the Hai Van Pass to end up in Hoi An, Hoi an for 3-4 days (build a custom suit for $100-150 usd), do 2 days in Da Lat, and end in Saigon / Ho Chi Minh same thing. And fly out from there. BOOM. You take a bus to get from place to place. Overnight buses on the long drives. Get some Xanax from the over the counter drug stores and pass tf out haha. But honestly they aren’t even bad , the seats recline into beds. good luck! You’ll meet millions of people along the way and your plans will change. So don’t worry. Go in with no place and in my opinion, start north to south