I did it a few years ago now. flew into Hanoi and a traveled south to Ho Chi Minh City using trains and buses. Just booked the flights and the first few nights accommodation in Hanoi after that just made it up as we went along. Really enjoyed Hoi An and Hue along with Halong Bay. Nah Trang wasn’t my cuppa. Would have like to get up to Da Lat in the mountains for the wine.
As peterfile says above really.