Begging & Postcard/Souvenir - Selling
Begging is not a common practice in Vietnam. Still you'll encounter some on the streets of cities like Ha Noi or Sai Gon, since Vietnam is a country with a lot of disabled people and it doesn't really have a proper social welfare system. 

Most beggars are not that aggressive, but they can be very persistent sometimes. At times, you'll see them as amputees or disabled, or with a needy small child/baby. Some of the beggars really need your help, some just pretend to need your help. If begging is a profitable business, then you'll see more and more beggars wherever you travel and you help to make foreign tourists targets for the beggars.

Many ruthless even use child/baby beggars to make money from tourists: say you give money to the child/baby, only a meagre part stays with them; the big part goes to the boss. Doesn't matter how much money you give the beggars, you are not going to change their life; believe it or not, the next day you come back, they'll still be beggars chasing you around. It's recommended that you buy some food/cakes for the disabled/child/baby and never give them money.

Souvenir/Postcard-selling boys and girls are more encountered than beggars and they are very persistent if not aggressive sometimes. The recommended practice is to say "No" (or "Khong" in Vietnamese) and keep doing your job. Or otherwise try to sell something back to them or try to befriend to distract them from focusing on their job.

The Vietnamese authority has done all possible methods for stopping people from hassling foreign tourists. In cities there are people dressed in normal clothes. If they catch any one selling things to or begging foreign tourists, they'll be caught and brought to the local police station. The first time caught, they'll be warned; the second time caught, they'll be sent to a so called "social welfare center" for 02 months; the 3rd time caught, they'll be sent away for 6 months. This definitely helps to improve the situation. Nowadays the travellers in Vietnam enjoy a much better peace touring the tourist areas in Ha Noi, Hue, Hoi An or Sai Gon.

Scams
Basically scams to foreign travellers involves taxi/motorbike-taxi or Xich-Lo (rickshaw) drivers. Many of them don't speak good English or pretend not to speak English. Before you hop onto the taxi/vehicle then it's one price, when you pay for the trip, it's another price. To avoid all troubles from this, use the metered taxis only. For long distances such as from the city to the airport, make it clear who'll pay for the road tolls. It's recommended to carry a note and a pen with you and write the prices down before hopping onto the vehicle.

Arriving at an airport, make sure prices are clear before you get into the taxi. It's reported that some ruthless taxi drivers know your name from the cheap hotel you've booked with and they'd offer discounted or complimentary ride to town. In the middle of nowhere you'll pay a hefty amount for the transfer otherwise you'll have to get off the taxi and get stranded there.

When you go to local restaurants, be sure the menus have prices. Without prices it's quite possible that you get a shock seeing the bills. This happens quite often even to the locals.

Going to bars at night, pay close attentions to free drinks! It happened to some travellers who enjoyed "free wine" from a bar in Nha Trang only to find out next morning that they'd been drugged the previous night. Please always keep it clear in mind that nothing is free!

Booking a tour from a cafe may make you a victim of scams. The reason is that there're hard competitions among the cafes to get the backpacker clients. Prices go down to the point that there's no profit, if not a loss. The result is that food quality goes down, accommodation quality goes down, bus quality goes down... It's no surprise if the bus get broken down and the passengers get stranded for a few hours; or bus with open-air only; or that the air-conditioner in your room stops working in the middle of the night; or the food doesn't make you feel alright after the meals...

Keep it clear in mind that all hotels have at least an extra key to your room and your belongings are dependent on the room keeper's/hotel owner's honesty. Some cheap hotels may  lack good people and some may want to make quick money. Hotels are not responsible for your belongings unless they are declared and deposited when you check in. Be sure you know what you are booking before a tour!

Traffic Accidents
You may have heard a lot about the traffic in Vietnam before arriving in the country, the reality is still beyond your expectation when you weave your way among thousands of mopeds and motorbikes blowing up their horns. Honking a horn means that they are coming and that you are in their way, but so many keep honking their horns at once informing you that you are in their ways! What should you do then? As a local you'd ignore it and keep walking at your own pace confidently. Keep moving slowly, carefully and confidently while in Vietnam!

If you happen to have a reckless driver while touring it's better to ask him to slow down, call his travel agent or get off the bus. As long as you are safe, it's never too late to travel to certain destinations. Be sure to be safe and sound, always!

Theft & Street Crimes
Serious crimes in Vietnam are not as rampant as in many other countries. Still it's never too much to use your common sense while traveling in this country. There are still pickpockets at train stations or crowded places in the country. Be aware of drive-by snatchers while in Saigon. Never have your belongings such as cameras, wallets, bags dangling loosely when traveling. Never swim with your belongings on the beach without someone reliable to watch them.

It may happen that you get off the train early in the  morning and some stranger comes up, hug and kiss and leave you...and so does your money.

Some advice to keep you from situations:

1. Never carry too much cash, jewelry on you while traveling: Most of your belongings should be put to the room safe or deposited at the reception of your hotel. Cash should be just enough for your daily expenditure. Your passport is needed if you stay overnight at another place or when you cash money from your credit card/traveller's cheques at a bank counter. No need for credit card if you intend to use ATMs.

2. Travel with a partner. This is especially useful if you get sick; or at train stations; or on beaches.

3. Never accept free food/drinks/cigarette from a stranger.

4. Acquiring insurance for your belongs while traveling. In the worst case that you have them lost or broken, your tour guide may help you to have certification from his/her travel agent/tour operator or from the local police.

Violence
Violence is not common among the Vietnamese, especially for you a foreign traveler. Still it's recommended you are ready with a big smile if you run into situation with the local boys at bars or in the streets. No one wants to loose face and bearing a smile doesn't make anyone loose face. In fact keep smiling is the best way to get out of situations in Vietnam.