It is called "Mã số thuế" (translated as "Tax Code") or "MST" (short version) and it is 10 digits long.
Also called "Mã số người nộp thuế" (translated as "Taxpayer Identification Number") according to image of some official certificate in https://easyinvoice.vn/ma-so-thue/ but Google tells that the above one is much much commonplace.
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