I spent months working in Vietnam working out of Hanoi and working in Highlands. (no experiences of the south as such) Try some local beer hoi, and a Cafe-no-da. Some of the street restaurants can be fine, try at least one bowl of Pho - noodle soup when you are there Bo=beef Ga=chicken I think i remember.
Lots of markets in Hanoi, went to opera house and saw cheap recital. Felt safe walking around at night. I read somewhere that police etc have been specifically told not to harrass tourists at the threat of losing pensions. Found people generally friendly.
Language isn't too bad either compared to say thai as its a romanised script and you can quickly pick up things, some french throwbacks too, cafe - omelette coffee.
If you can try going to the silk / pottery villages. Saw some amazing hand painted pottery / tea sets.
Highlands was quite interesting, went to Hue and was lovely temple complex but if anything you were allowed to walk all over it which isn't good for its preservation. Sea was weird in how warm it was, strong onshore currents to pushing you back onto the beach!
Taxis -i think they all have meters now?!- i used the green and white little daewoos a lot. other wise make sure agree price up front (I was done once)
Look for advice on boat trips, I think people should be vigilant about night boat trips, look for reviews of safety before you go.