Although this post is a little old, I just wanted to add my two cents in case anyone else has the same concerns! I spent a month travelling down Vietnam with a female friend in 2012 and had a truly wonderful time. I cannot recommend a trip to this stunning country enough, the people and atmosphere are so relaxed, not to mention the incredible architecture, history and delicious food.
Moreover, at no point did we ever feel unsafe, even tottering around Hoi An at four in the morning more than a little tipsy. The only thing I would say is to compare prices in a few places before you agree to buying anything, but as everything is so cheap compared to Western prices, even paying slightly over the average isn't a huge deal in my eyes.
At the risk of sounding like their tourist ambassador I have to say that if you are thinking of going to Vietnam, BOOK YOUR TRIP NOW, it will give you memories that you will look back on fondly and cherish for years and years to come.
Nat x