I have been to northern, southern and central Vietnam, my recommendation is to travel a few cities/ town, spending a few days each. Day trip to far places isn’t worth it if the travelling distance is more than 2.5 hrs long
Hanoi, Halong Bay , Sapa/ Ninh Binh, Hanoi- easily 7 days or more
Ho Chi Minh City, Mui Ne/ Phu Quoc, Ho Chi Minh city - easily 7 days
Danang, Hoi An, Hue, Danang - easily 7 days too.
when you land in Hanoi, Ho Chi Minh city, Danang, or Hoi An, find a tailor and tailor some trousers (3 days turnaround) or suit ( measurement to collection 7 days or more for half canvassed suit). Don’t go for 24 hr or 48 hrs fused suit. Their workmanship is excellent for relatively low prices. if you bring your own imported branded fabrics, workmanship for half canvassed suit is less or equal $300 or 5 million dong. I have tailored many times in Hanoi, Danang, Hoi An and Saigon. Hoi An can make suit in 48 to 72 hours, but quality is lower. I sell branded fabrics in my signature link below.
the authentic cuisine in each city is different. Danang ( banh xeo) is different from Hoi An ( Mi Quang, cao lau, chicken rice, banh mi) and Hue ( Bun Bo Hue) , Saigon (com tam, Oc Dao - snails), Hanoi ( banh cuon, Bun Rieu, bun Bo nam bo, bun Cha ), Sapa ( Thang co -horse meat) take your time to savour the local dishes, some cannot find in Singapore. Even northern pho is different from southern pho.
don’t rush to chop stamp only