There is some contradictory information about that. Please download the new documentation set (V2).Here you can find all system exports per country/year.
SA-75MK Dvina “three van” (SA-2B/F)
Vietnam 1965-16+1*, 1966-18+1*, 1967-42+2*, 1968-4, 1969-2, 1972-12
* means training system
S-75M* Volhov (SA-2E)
Vietnam 1979-8, 1980-3, 1982-3, 1985-4, 1986-8, 1987-10
S-125 Neva (SA-3)
Vietnam 1972-4, 1973-8, 1979-9, 1980-3, 1981-1, 1982-3, 1984-3, 1986-6, 1988-3