How do you find out whether or not certain over the counter vitamins in the U.S. are legal or bring into another country?
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As far as I'm aware, vitamins are always legal to bring into another country. Maybe you are talking about something else? Unless it's a prescription medication, in which case you just need to bring the prescription with you, you should be fine to carry any sort of food supplement in with you.
If you want to be absolutely certain that something is not prohibited, then check with the customs of the country in question. e.g. for Vietnam, check here: http://www.customs.gov.vn/English/Default.aspx