Experimental data on vapor phase second virial coefficient isotope effects (VCIEs) are reviewed and then interpreted using the general theory of isotope effects. Useful correlations are developed between -δ(ℬ - bo)/(ℬ - bo) = (-VCIE) and [ln(fc/fg)]*, where [ln(fc/fg)]* is the reference condensed phase reduced isotopic partition function ratio, and ℬ is the second virial coefficient, bo = 2πσ3/3, σ is the Lennard-Jones size parameter, and δ denotes an isotopic difference, light-heavy. [ln(fc/fg)]* can be straightforwardly obtained from measurements of vapor pressure isotope effects for TR = T/TCRITICAL < 0.7. We show (-VCIE) = ln(fp/fg2) where ln(fp/fg2) is the reduced isotopic partition function ratio associated with the equilibrium between isolated gas-phase monomer species and interacting pairs. At temperatures well removed from crossovers in ln(fp/fg2) or [ln(fc/fg)]*, ln(fp/fg2) = (0.4 ± 0.2) [in(fc/fg)]*.