I went to Vietnam with Just You in 2011 and it was one of the best trips I have been on. However you do need to be very careful with your belongings and I would defiantly recommend some kind of security wallet worn under your clothing.
Crossing the road in Vietnam is a very interesting experience and on the last night of the trip I went on two women had their bag stolen by people on motorbikes. If your passport is stolen you cannot return home until you get a temporary replacement and this can take a few days.
Please do not let this put you off this holiday. It is a wonderful place to visit but you just need to be a little careful.
David