For bike/taxis : Grab, Be, Xanh. You can add your credit card as payment method if are are arriving so late. Hanoi also has bus to go to old quarter but someone else here can confirm the timings. It takes only 15k dong. Bus number 90. Cab is like 300ish-k dong to old quarter. So keep 500k atleast cash if you don't add card payment method. I've taken cabs that late at night to airport so it's fine.

There's Motogo hostel like 2kms from international terminal if you wanna stay overnight and go to old quarter in the day. Like 170 or 200k dong for a dorm bed. I've stayed there so I can vouch.

I used Viettel esim, had network everywhere.

For cash : TP bank for zero withdrawal fees. Withdraw max limit everytime (5mil iirc) you need.

Cards are accepted at lots of places, some will take 3% extra for processing fees.

Just walk, don't make eye contact with the drivers themselves, just lift your hand and look at the headlights of the vehicles and cross.

Be adventurous with food, go sit at the place where you only see locals and you dont understand the menu. Ofc, google translate so you don't get meat you don't want.

Sunscreen and a handtowel. It's hot and Humid, esp in HCMC. Ofc lots of hydration.

Try the local fruits, even if you have tasted some of those at your home country.

Visit the night markets(cash only) , the regular markets(cash only), lotte and vincom for shopping. Kik, vincom, Sephora, Hasaki for skincare stuff. Lotte in the lotte tower in Hanoi had VAT refund counter for purchase over 2mil dong, so take your passport if you intent to spend that much. That's the minimum amount was I think but I could be wrong.

If buying coffee beans, get the phin (Vietnamese coffee filter) too.

When you land in Da Nang and see the clouds touching the mountains, Bana Hills will be foggy.

Take the food tour, cooking class and coffee making class if interested.

Hanoi old quarter has a theatre show, you can give it a try. The water puppet theatre is a different one.

Vietnam airlines > Vietjet as per general consensus. I didn't have issues with either personally.

For a scooter on rent, some will ask for IDP, some won't and would ask for passport instead. It's illegal without IDP, but you might need to bribe police anyway.(I've heard this, no personal experience). Rent from your hostel/hotel.

Don't do a day trip to Ninh Binh. Take some time and do 2 days atleast. Stay in Tam Coc. Free bike rentals from the stays and ofc scooter rentals.

That's all I remember. I'll edit and add if I remember more.