try to avoid walking around alone at night in cities as apparently attacks on westerners are escellating in many countries, especially muslim countries. Places like Kenya are really bad at the moment.
Basically its muggings etc and the people are not hurt but its still a very scarey experience.
Let me give you a piece of advice, while travelling keep 2 wallets.
One wallet is your main wallet with the most money in it and the other is your 'muggers wallet' with a bit of money in it that wont hurt too much to lose.Some one jumps you you give em the muggers wallet with no questions asked.Let the fool take it

and you can go spend your cash in a bar to calm yourself

I dont mean to worry you or anything but be prepared maniac and be sensible and im sure you will have a fantastic time out there.Most vietnamese are incredibly friendly and im sure you'll be very glad you went.