Official Vietnamese government figures, independent demographic reconstructions and various American and other estimates disagree sharply on how many Vietnamese died, with published totals ranging from under one million to more than three million; the gap widens further once Cambodian, Laotian and American deaths from the wider conflict are added or excluded. Wartime record-keeping was incomplete on all sides, some published Vietnamese figures are disputed as politically motivated, and demographic back-projections depend on population and birth-rate assumptions that are themselves contested.
What would resolve this An independent demographic reconstruction using census and vital-registration data from before, during and after the war, cross-checked against Vietnamese, American and international records, which has not been completed to a scholarly consensus.
OpenHistorical demography and Southeast Asian studiesWikipedia: Vietnam War