All of the recruitment agencies in HCMC have websites with job listings and career opportunities but it is definitely easier to look for a job once settled in. You will usually be required to upload your CV and fill out a form summarising your job skills upon registering. If possible, call them directly and schedule a meeting. One of the difficult aspects of looking for a job on these sites is determining if a job is available for an expat or only locals. A good tip is to see if a position is solely for an expat, and whether English is required as a native tongue. None of the agencies charge fees for their services since they collect a commission from the company that employs you. Check the sites of large HCMC-based companies for vacancies and updated listings for opportunities to work in Saigon.
For information on working in Vietnam go to Culture Clash: Multinational Working