We calculate virial coefficients of gas mixtures to demonstrate their use for interpreting gas-saturation measurements of the vapor pressure of low-volatility compounds. We obtain coefficients from molecular models, via calculation of mayer integrals that rigorously connect the models and the coefficients. We examine He, CO2, N-2, and SF6 as carrier gases, and n-C14H30 and n-C20H42 as prototype low-volatility compounds, considering both united-atom (UA) and explicit-hydrogen (EH) alkane models for them. Both the pure virial coefficients of every species and the cross-coefficients of each gas with n-c(20)h(42) are calculated up to third order; cross-coefficients of SF6 with n-C14H30 and all EH-based coefficients are given only to second order. Using these coefficients, we calculate corrections to the vapor pressure of n-C20H42 at 323.15 K for all four carrier gases. With the corrections, the derived vapor pressures in He, CO2,N-2 carrier gases are in excellent agreement, resolving most of the variation observed in apparent vapor pressures when gas-phase nonideality is neglected. Results are less satisfactory for SF6 as the carrier gas. We also calculate corrections to vapor-pressure data for n-C14H30 at (283.15, 293.15, 303.15, and 313.15) K in an SF6 carrier gas.