Transportation
Note that cyclos are banned from some streets in the city centres, so don't be surprised if you're taking a slightly roundabout route. Also, it's not a good idea to take cyclos late at night in HCMC as some people have been mugged.
Always agree on a price before setting off, preferably by writing it on a piece of paper. Make it clear whether you're bargaining in dollars or dong and whether it's for a single or return journey. It helps to have the exact money ready at the end to save any arguments or hassles over change.

Motorbike taxis (called xe om or Honda om) come in handy in country areas, especially in the mountains. You'll also find them in the main cities where they are a shade cheaper than a cyclo and faster, though perhaps even more scary in the chaotic traffic. The same rules of bargaining apply.

Metered taxis now operate in HCMC, Hanoi and other major cities. You'll find them outside the main hotels or cruising the streets, though you can also order one by phone. Rates for short journeys within the city centre are around 10,000d per km with a flag fall of 12,000VND. If you can only find an unmetered taxi, make sure to agree a price before jumping in.
1. Transportation in/out from airport to downtown Hanoi and Hochiminh city
HCMC's Tan Son Nhat airport lies about 7km northwest of the city centre. The best way to get into the city is to take a taxi. Ignore the expensive airport taxi booking desk and pick up a metered taxi outside the terminal (you might have to insist they use the meter; if not agree a price with the driver before setting off). The journey should cost around $7-10. I gather that there's also an airport bus ($2), but that it only runs between the airport and the Vietnam Airlines office on Nguyen Hué. Note that some of the more expensive hotels provide airport pick ups, so ask about this when booking your room.

Hanoi's Noi Bai airport is 35km north of the city. A taxi into town should cost around $15-20. You can either pay at the taxi booking desk inside the airport building (make it clear your paying for a taxi and not the minibus), or find your own taxi outside. A cheaper option is the Vietnam Airlines minibus ($4), which drops you outside their office at the south end of Hoan Kiem Lake, though they may take you to your hotel for an extra $1. Tickets are sold inside the terminal building.
2. Train
There are five classes on Vietnamese trains: hard seat, soft seat, hard sleeper, soft sleeper and "super berths" (soft sleeper with air-con), though only the long-distance expresses offer the full range. For short journeys hard or soft seat carriages are fine, though most Vietnamese people can only afford hard seat so these carriages tend to be packed out.

For longer journeys, particularly overnight, try and get a sleeping berth. Hard sleeper consists of six bunk beds in one compartment (two tiers of three). In most cases the seats have padding and it's reasonably comfortable; the exception is the night train to Lao Cai, when you just get a hard wooden bunk and a mat to sleep on. Prices for the bottom bunk are highest and the top bunk is the cheapest, partly because you have to climb up and also because there's very little space between the bunk and the ceiling. Soft sleeper compartments have four bunks (two tiers of two) which are all priced the same. Note that sleeping berths get booked up well in advance, so it's best to make your reservation as early as possible.

Meals are provided on longer journeys, though you may prefer to take your own supply of snacks, fruit and bottled water. You can supplement these when vendors come on the trains at stations to hawk their wares.

As far as safety goes, the trains in Vietnam are fine. However, theft is becoming more of a problem on some routes, particularly the night trains from Hanoi north to Lao Cai. It's wise to ensure your luggage is locked and safely stowed away, don't leave anything lying near an open window and always carry your valuables with you. In the sleeping carriages the bottom seats lift up and you can put your bags underneath, so these berths are quite a good idea. The only downside is that everyone will be sitting on them during the day - which means you can't stretch out if you want a nap. The top berths have access to a useful storage space above the door