Two weeks gives you enough time for all three regions plus both mountain and beach experiences. The route is full, but it stays workable by using flights for long jumps and protecting a few travel days from excessive sightseeing.
Hanoi
Start with two nights for the Old Quarter, key historical sights, coffee and a proper introduction to northern Vietnamese food.
Mountain landscapes
Travel to Sapa for valley scenery, villages and Fansipan. Two nights gives you some flexibility if mountain weather changes.
Return to Hanoi & reset
Travel back toward Hanoi and use the evening as a buffer before Ha Long Bay rather than forcing another major attraction into the day.
Ha Long Bay
Choose an overnight cruise if the bay is a trip highlight. A day cruise is possible, but two weeks gives you more room to experience it slowly.
Fly to Da Nang
Return from the bay and move south. Keep the evening easy rather than building a tight attraction schedule around a multi-stage travel day.
Da Nang highlights
Choose Ba Na Hills for a full attraction day, or Marble Mountains plus My Khe for a more flexible city-and-coast day.
Hoi An
Move to Hoi An for two nights of heritage streets, regional food, countryside and a slower evening pace.
Ho Chi Minh City
Fly south for two days of city history, markets, cafés and food. Use one day for central sights and keep Cu Chi optional depending on your interests.
Beach finish
Fly to Phu Quoc and finish with two slower days. Keep the schedule light enough that the final part of the trip feels like a break rather than another checklist.