The piece you're looking for refers to Decision 203/QD-EVN (Quyết định 203/QĐ-EVN), which was issued by Vietnam Electricity (EVN) on October 27, 2020, and came into prominent use throughout 2021.
Given your use of "QĐ-EVN" (Decision by Vietnam Electricity – EVN), you are almost certainly referring to a technical norm for distribution grid losses or electricity meter installation. Based on EVN’s internal documentation, the most likely candidate is Decision 203/QĐ-EVN dated December 3, 2015, which regulates the Allowable technical grid loss rates for distribution systems. There is no public 2021 update to this specific decision. dinh muc 203 qdevn pdf 2021
QĐ 203 provides base norms for the Delta region (Red River Delta). If your site is in the Highlands or Northwest, you must apply a terrain adjustment coefficient (hệ số khu vực). These are not in the 203 PDF but in Circular 02/2021. The piece you're looking for refers to Decision
The document commonly referred to as "Định mức 203" is officially titled: Quyết định số 203/QĐ-EVN dated February 4, 2021. There is no public 2021 update to this specific decision