If you work for a business in Vietnam, including all the International schools and universities, you are expected to have 35% taxation, plus health insurance, automatically deducted from your wages. Your employer is officially required to register you as a foreign worker, arrange your work permit, and pay these deductions to the local government on your behalf. There does'nt appear to be any way to get tax refunds from the system, unlike other countries, if you are not here permanently.
Having said the above, there has recently (early 2010) been a lot of problems with local and international employers employing foreign workers, especially teachers, and not carrying out the official registration. They will fill in forms, make the deductions, but not register workers or pass the m oney on to the authorities. Usual Vietnamese scam !
The reasoning, apart from scamming foreigners, is that local businesses are reluctant to let government know how much they are paying people, how many people they employ, and thereby how much money they are making. If the authorities get to know this, they are very likely to increase the tax rates for the local business!! So for a long time local business have been working this scam.
Beware of any local business that offers to employ you without deducting tax, or doing the proper registration. If you get caught you will suffer instant expulsion from Vietnam, they are getting that tough on illegal foreign workers.
Still, there are many foreign teachers who have one or more small official jobs, but can make significant money doing private tuition, mainly teaching English. private familieis will pay well for good quality English teachers to help their teenagers get advanced tuition.