Day 1: Arrive in Hanoi in the morning and visit the city
Day 2: Day trip to Ninh Binh
Day 3: Leave in the morning for Ha Long Bay (1 night boat tour)
Day 4: Han Long Bay and return to Hanoi
Day 5: Bus to Sapa
Day 6: Sapa
Day 7: Sapa
Day 8: Bus back to Hanoi
Day 9: Hanoi (or you can leave the day before directly from Sapa to Hoi An)
Day 10: Bus to Hoi An
Day 11: Hoi An
Day 12: Bus to Nha Trang
Day 13: Nha Trang
Day 14: Bus to Ho Chi Min
Day 15: Day trip to Mekong delta
Day 16: Flight back home

A few things to note here:
- All cities in Vietnam are very well connected by bus, so every time you can I would suggest you to book a bus and not a flight, they are also extremely cheap and eco-friendly. Another big plus, you can find most bus routes going overnight, which can save you from losing a day travelling.
- In Ha Long Bay I would suggest you to go for a 1 or 2 nights boat trip. No more than that. This is probably going to be the most expensive activity in Vietnam (as everything else is extremely cheap).
- I only left 2 nights in Nah Trang, but if you have more time and want to relax I would recommend more days.
- In this itinerary you won’t see much time in Ho Chi Min, that is because I didn’t like it personally and I wouldn’t dedicate much time to it.