The best answer to your question will come from an Embassy or Consulate of the Socialist Republic of Vietnam. I am unable to find (in English) any "live" websites from those entities.
Whle only a secondary source, and directed only to US Citizens (not citizens of other countries), this US State Department webpage under "Entry, Exit and Visa Requirements" contains the following language describing visas issued by Vietnam:
Visas: When you apply for a visa to enter Vietnam, be sure to request the visa category that corresponds to your purpose of travel. Please refer to Vietnam’s Ministry of Foreign Affairs website for information detailing visa categories. If you plan to work in Vietnam, you must obtain a work permit before applying for your visa. If you change the purpose of your visit after you have received your visa, you must obtain a new visa outside of Vietnam appropriate for your new activities before beginning those activities. Please consult the Embassy of Vietnam website for more information.
Vietnamese governmental rules regarding work are unlikely to be different for different other nationalities. I conclude you are correct (one can't work for years on a tourist visa), and your friend is incorrect.