Every country operates a little differently, and sometimes it helps to know a little bit of what should be very basic information before you travel to a new location.
The official currency of Vietnam is the Vietnamese Dong, of which, roughly 22,000 converts to $1USD, depending on conversion rates at the time of exchange. The US dollar is also widely accepted all over Vietnam, and most touristic establishments will accept payment in both currencies. Some places will take credit or debit cards, but a lot of establishments will only accept cash, so make sure you have some with you before you leave.
Prices in Vietnam are often not fixed like they usually are in Europe or America, and so often the locals will inflate the price for tourists. There is no point in pretending you're a local unless you can speak fluent Vietnamese, so be prepared to haggle and to bargain prices, and always check that vendors give you the correct amount of change that they owe you after agreeing on a price.
When it comes to paying for transportation, make sure you purchase your tickets directly from the train station and not from an agent or a hotel as they tend to overcharge as they want to make a profit from your business. Vendors will often say anything to obtain your business, so even if they insist that it is the price of the ticket, it can be purchased at the train station for the actual price, regardless of what you are told.
In Vietnam, they drive on the right-hand side of the road, although roads, particularly in cities, can be very chaotic and often motorbikes and scooters can be seen darting between traffic in brave maneuvers from all directions. It seems like organized chaos, but the locals manage to navigate with ease, however you should consider whether you think you are capable of driving in such manic conditions before you hire a car. Once you are outside of the cities, things calm down considerably, but road conditions are not always great. Traffic accidents are the most frequent cause of tourist deaths in Vietnam, so if you are driving, be careful.
Most Vietnamese who work in the tourist industry will have a good enough grasp of the English language, however as you find yourself in more rural communities, you might struggle to find people who can understand English. These are some key words and phrases that you should learn in the local language of any country that you are travelling to as a show of respect, even if only to be dismissed and spoken to in English. Learning these will actually mean that you know more Vietnamese than 90% of the travelers in Vietnam. There is a pronunciation guide in brackets to help you say the phrases correctly.
chào b?n (te-ow ban)/alô! (ah-low) - Hello
t?m bi?t (tam bi-ey) - Bye
cám on (cam uhn) - Thanks
Obviously, you can go much further and learn entire phrases, but even just saying chào b?n will be well received by the locals and create a good impression.
Visas are often required by visitors, and you should check with the official websites before departing for Vietnam if you are required to purchase one. This is taken very seriously at the airports, and so make sure you check all your information matches up correctly, make sure you have an accommodation address to provide to the immigration officers, and make sure you treat them with respect.
Food poisoning is quite common for tourists in Vietnam, so make sure you eat at trustworthy sources. Soup can often be the cause as it has been sat for a long time, so ensure it is boiling when it arrives and if it is not, politely send it back. Busy restaurants have a larger turnover of customers and therefore have food prepared more frequently, and are slightly less risk than quieter restaurants.
Vietnam is a fairly safe country to travel but petit thieving is still quite common. Make sure you keep an eye on your belongings on trains and busses, particularly when travelling overnight. Don't leave your money or your phone on the table at restaurants.
Vietnam is actually quite a big country, and it takes longer to travel around than maybe you think before you start your trip.