More detail about differences in the URBAN variable across samples can be found on the comparability tab. Specifically for Vietnam samples: “In 1989, places designated as urban included all cities and provincial towns. A district town was considered urban if it was an administrative or industrial center of the district, it had 2000 or more population, and over half its workforce was outside of agriculture. In 1999 and 2009, urban areas include urban districts of cities, urban quarters and towns. All other local administrative units (communes) are rural.”
Additionally, regarding more specific information about geographic area identification in IPUMS International, check out the country specific geography variables. (See, e.g., GEO1_VN1989, GEO1_VN1999, or GEO2_VN2009) In Vietnam, the “level 1” geography is the province and the “level 2” geography is the district. Finally, a complete list of all geography variables available in IPUMS International is available on this page.