Women traveling alone in Vietnam face no particular safety issues, and common sense should keep anyone safe. If alone, female travelers in Vietnam (everyone, really) are repeatedly grilled about their marital status and, if single, pitied -- a bit exhausting after a while. Some women even revert to wearing wedding bands to end those conversations before they begin. No particular vigilance is required for female travelers, as violent crime is minimal in Vietnam, but the usual precautions about walking alone at night and hitchhiking certainly apply, as anywhere. "Catcalling" happens but is rarely sinister or followed by any action.
For general travel resources for women, go to www.frommers.com/planning.