Three-body electron attachment to O₂ molecules and electron detachment from ions have been theoretically studied in vibrationally excited oxygen and O₂-containing mixtures. Assuming that electron attachment and detachment proceed via the formation of vibrationally excited temporary ions, the rates of these processes were determined on the basis of the statistical approach for the vibrational transfer and relaxation in collisions between ions and O₂ molecules. The calculated attachment and detachment rate constants turned out to agree well with available measurements in unexcited oxygen. This method was extended to calculate attachment and detachment rates in vibrationally excited oxygen. It was shown that the effect of vibrational excitation on electron detachment is profound, whereas attachment of low-energy electrons to vibrationally excited O₂ is inefficient. The calculated vibrational distribution of stable ions turned out to be non-equilibrium in an excited gas and the effective vibrational temperature of the ions was much lower than the vibrational temperature of molecules. An analytical method was suggested to determine this distribution and the effective vibrational temperature. The calculated rate constants were used to simulate the formation and decay of an electron-beam-generated plasma in N₂ : O₂ mixtures at elevated vibrational temperatures. The calculations showed that vibrational excitation of molecules leads to orders of magnitude increase in the plasma density and in the plasma lifetime, in agreement with available observations.