Here are a number of simple tips and facts in no particular order: Get from real travellers and add new more tips

  • First and foremost, be forewarned that Vietnam is a hectic, noisy, relentless travel experience.  The country is crowded and frantic.  If you are looking for a mellow vacation, better to head to Laos or Cambodia or even Thailand (as built up as it is).  Vietnam is one of the most "in your face" places you may ever visited.  Having said that, it is an extremely interesting country with an unbelievably varied countryside, culture and people.  Every place you visit -  north to south is unique and very different.  You will not forget the experience.
  • Saigon (HCMC) is probably the least interesting of the major cities in Vietnam.  Hanoi and Hue and even Danang are all probably more interesting.  If you time is tight, shorten up the HCMC portion. You may find it more interesting to leave the city to tour the Mekong Delta.
  • Similar to foregoing, if you don't have time to visit whole country, split in the middle and stay in the northern half - Hoi An north to Hanoi.  Hanoi, Hue and Hoi An are the three most interesting cities (leaving aside beach resorts).
  • Hoi An is the nicest city to visit - everyone says they wished they spent more time there.  Give yourself at least three to four days there..
  • For history and culture buffs, Hue is the city to visit.  Rent a bike and ride all around inside the Citadel, one of the high-lights.
  • There are tons of travel agencies all through the country so if you want to go without pre-booking everything, you can do it.  Also, use the private cars to get to places that do not have flghts and to avoid long, hot, crowded bus rides. They are reasonable and very easy to arrange.  Your hotel can arrange for you but you can cut 10 - 15% off by doing yourself.
  • If you want to travel light from home, you can get everything you need cheap and in the street markets in all of the major cities so don't worry about trying to bring everything you need. For example, you can buy copies of all of the major guide books on the streets for 100.000/120.000 dong, same things with english novels.
  • Similary, you can buy hats, flip flops, sneakers, t-shirts and pretty much everthing you need to wear but even in big cities the price are not very cheap anymore, just less than in Europe,(so you could literally come with a half filled bag, buy what you need as you go, and toss when you are finished as they may be usually in poor condition).
  • Used Vietnam Air for several internal flights and booked online and have no problems.  The planes are clean, nice and on time.
  • Don't drink the water.  Bottled water is cheap and everywhere.
  • No big problem finding ATMs, see Banks and money.  No need to carry cash.
  • While the traffic looks crazy, you can rent bikes (you need a Vietnamese driving licence) in a lot of locations and actually find riiding in the middle of the madness of a million scooters not to be as insane as it looked. It is a nice way to get around but very dangerous.
  • Taxis are rarely a problem, even though there seems to be a lot of warnings.  Just ask them to use the meter. 
  • Dalat has a reputation for tackiness, and some aspects of it are, but in and around the town are many places worth the visit.  
  • Hanoi is very interesting and try to stay in the Old Town (36 streets). 
  • Halong Bay was worthwhile, but be prepared for rain as it is notorious for that.  Also, would recommend using one of the nicer charters as the boats have less guests and nicer accomodation.
  • To fly to Hoi An, you must fly to Danang and take a 30-45 minute taxi, bus or car ride to Hoi An.  Hue has its own airport so while some get there by flying to Danang and then travelling by road (2 1/2 to 3 hours) or train, you can fly directly to Hue.  You can take a private car to get from Hue to Hoi An and vice-versa.
  • Be forewarned about bus rides and private cars, while the distances don't look very far, travel is very, very slow.  Count on about an hour or so for every 30 - 40 kilometers by car or bus. Sleeper buses can be uncomfortable for larger/taller sized travellers and some sompanies have poorly maintained vehicles and dangerous drivers.  Get recommendations for reputable companies.
  • An annoying habit to check when booking is that they will try and make you pay for the credit card fees, somwehere between 2.5% to 4% if you want to pay for things by credit card.  Not a big thing, but keep in mind that it will add to your expense.
  • Learn a few words of Vietnamese - it will make for a much happier, more pleasant experience. 
  • Don't trust the vendors seller in Hanoi Old Town, you must bargin clearly before use.
  • Best time to visit Hanoi is autumn.
  • Best "Pho" in Hanoi is "Pho Bo Gia Truyen" at 11 Bat Dan street; 49 Bat Dan street is great too
  • Try a cafe in Hanoi: "Lam cafe", "Nhi cafe", "Giang Cafe"
  • Try to get offline maps. if you have an iPhone "offmaps 2" is a great app to have by your side. For around $7 you can download unlimited maps for many different cities and towns that operate offline (with out data usage, so you could use your IPhone without a sim as long as it has GPS enabled).