It sounds funny when I say that begging is a kind of professional business. Yes, It's happening in Viet Nam. There are even people who hire kids to be beggars to earn money for them. Vietnam's government has been trying hard to stop this practice but it seems hard to beat. Lots of people have been arrested, however.
Many kids have been cheated and then kidnapped from the countryside and even in the big cities to be money slaves. Beggars young and old are commonly seen wandering around try to get money, especially in the old quarter. Some young people who are completely healthy but lazy try all kinds of tricks and stories to get money. If they get refused, their attitude changes immediately and they might shout at you or throw something to you.
Some pretend they are sick with diseases like AIDS to get sympathy. Besides these pretenders, there are also lots of people with real reasons to beg but the percentage is very low. Most poor Vietnamese would rather do some kind of job to earn a living.