Classic North-to-South itinerary
15 days (fast): Hanoi 2d, Halong Bay 2d (junk cruise), Ninh Binh 1d, flight to Hue 2d, Hoi An 3d, Nha Trang 1d, Ho Chi Minh 2d, Mekong Delta 2d.
21 days (ideal): add Sapa (homestay trek, 3d) up north, Dalat (cool climate, waterfalls, 2d) centre, Phu Quoc island (beaches, 3d) south.
30 days (immersive): add Ha Giang (scooter loop, 4d), Cat Ba archipelago (Halong alternative), Con Dao.
Direction: classic North-to-South (milder climate as you descend in winter) or South-to-North (in summer, avoid southern heat).