You can book a city tour in HCM for 1 day.
Or Book a Mekong Delta Tour too many options for you.
Ask the receptionist and take a map with full detail.
District 1 is a city center and easy to find a restaurant,hotel,v.v...Please deal that you have a best prices.
And Nightlife you can Go to Forgeiner Town in District 1.
Thats enough for you discover about HCM.
No need to stay longtimes in HCM.
Best beach Place and Old Town and you have more knowledge about history in Vietnam.
In the North of Vietnam is Hanoi
You can enjoy many food in there(Bun cha Hanoi,Bun Dau nam tom and Noodle( Phở)),
That a special taste.
If any question about HCM more,please send email to me : firstname.lastname@example.org
I will help you anytime.
FYI.Great day to you.
HCMC is great base for Mekong Delta, CuChi Tunnels, Markets and the War Remnants, all worth visiting.
Hanoi is a beautiful town with so much to see and experience. Halong Bay I believe is a must also.
Mui Ne is amazing, and we loved our time there. Swimming, watching the Kite Surfers, and mostly relaxing.
All good choices if you can fit them in.
For your 3 week trip, you can visit other places apart from the destinations you mentioned. From Ho Chi Minh City, you should take a tour to Mekong Delta
However, the itinerary also depends on when you visit Vietnam because the weather is different among places. Check out the weather before your trip.
HCNC is definitely worth a visit. Some excellent things to see and do.