Thank you so much Beanstalk6 and Paladin!
Appreciate your help! Will adjust the itinerary accordingly. I don't intend to have a very rush trip. But to enjoy Hanoi/HCM and maybe Danang.
Actually can extend even Hue, Danang or Hoi an, e.g, you can check in only from 2pm, if you reach in the morning, are you going to bring your luggage to tour imperial palace, imperial tombs, Bana Hills, Hoi An coconut forest, or you are going to put them at hotel first and then go out to visit the places? Bana hills is at least 40mins from Danang city. Hue imperial tombs 40 mins from imperial palace. There are 3 tombs sites if I remember correctly.
Bana hills ( golden bridge, wine cellar, buffet lunch, amusement park, exhibition, shows, parade, garden, etc) can go up at 9am and come down at 6pm, and may not be enough still.
a lot of people go Hoi An never venture outside the old town unesco sites. There are plenty of attractions outside old town like coconut boat ride at coconut forest, terracotta park and learn pottery making at the village beside, Hoi an memories show and theme park, vin wonders Hoi an for river safari and different village huts of different provinces etc. Hoi an has its own cuisine too. Hoi an also has rice fields and beach. At night, can take boat along river and see lanterns.
https://maps.app.goo.gl/U781JM6XMGjjZ76s7?g_st=ic
danang has wonderful beach, seafood, Che ( dessert) , restaurants
besides local food and you will not repeat same dish in 21 days from north to south, these international restaurants are also recommended if you want something different
HCMC - Japanese restaurants like izakaya at Japan town - many of them and quite authentic with unique food like special rice balls.
https://maps.app.goo.gl/cQFBQpdEyM4Gcu7Q8?g_st=ic
Danang - French restaurants like La comptoir
https://maps.app.goo.gl/vZxqimcj5vHsA1c87?g_st=ic
hanoi and Hoi An - Korean bbq restaurants
https://maps.app.goo.gl/mLuJr6quMDYhWFX4A?g_st=ic
because Vietnam was colony of French, and now has huge investments from Japanese and Koreans with large communities in these cities.
vietnam north cuisine (chim quay (roast pigeon), bun Cha, banh cuon, egg coffee, banh canh ghe, rice crust (ninh binh), goat meat (ninh binh) , horse meat (sapa) , bun Bo (dry or salad) ) etc
vietnam central cuisine ( Mi Quang, Cao lau, banh xeo, rose dumpling, bun Bo hue, seafood, banh beo, chicken rice) etc
vietnam south cuisine ( com tam, oc ( snails and shellfish), etc
north pho is different from South pho
banh Mi is everywhere but I find Hoi An the best. (MDM khanh)
spring rolls everywhere,
hanoi and HCMC have Chinatowns with some excellent dim sum restaurants too.