Most of the Vietnamese and asian culture, I will let you discover for yourself, but here are a few tips
Hello = Xin Chao (sounds like sin ciao)
Thanks = Cam on (sounds like Cam Uhhnn)
Yes = Vang (sounds like va-ong)
No = Khong (sounds like hum)
Many things don't sound like how you would read them and pronounce it in English.
Hanoi is a pretty safe place in general. Saigon less so (in case you're going). You can wear/take valuables pretty much anywhere in Hanoi, but be vigilant as usual in any big city.
When negotiating anything in Asia, remember many people will try to take advantage of you.
Traffic is dangerously hectic here. Always! be on the lookout and very careful. You may need to brave walking in front of people, but always try to make sure motorcyclists and drivers notice you. Be big. While you're looking one way, don't look too long without looking the other way.
Uber is available and trustworthy. It also means you don't need cash. I highly recommend using Uber. Grab is a similar app here in Hanoi, but where you need to pay cash at the end of the trip. Many drivers will try to call you because it is a busy city but that's not an option for most foreign phone numbers. Just notice the license plate's last two digits in your app and be on the lookout.
If you don't have WiFi nor a data connection, then use taxis with a meter. Most negotiated rates are a scam. The green cab company called Mailinh is the most trust worthy, and generally anything you get from a hotel. If drivers try to negotiate a rate, just say no and get out.
- PLACES TO SEE, EAT, DRINK
Tripadvisor and ratings are not always dependable here because they're hacked by many friends and competitors posting fake reviews. Still they're worth checking.
While not a definitive list, we have put together a map/list of about 50 hangouts in Hanoi...mostly to eat & drink. This should open in the Google Maps app. On the Google Maps site in your desktop browser, it will only show about 20 places unfortunately, so use an iOS or Android device.
Note that the Google Maps app lets you download a map into your phone and use it offline with GPS.