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 -Delta (B - b(o))(B - b(o)) = (-VCIE) and [ln(f(c)/f(g))]*, where [ln(f(c)/f(g))]* is the reference condensed phase reduced isotopic partition function ratio, and A is the second virial coefficient, b(o) = 2 pi sigma (3)/3. sigma is the Lennard-Jones size parameter, and Delta denotes an isotopic difference, light-heavy. [ln(f(c)/f(g))]* can be straightforwardly obtained from measurements of vapor pressure isotope effects for T-R = T/T-CRITICAL < 0.7. We show (-VCIE) = ln(f(p)/f(g)(2)) where ln(f(p)/f(g)(2)) 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(f(p)/f(g)(2)) or [ln(f(c)/f(g))]*, ln(f(p)/f(g)(2)) = (0.4 +/- 0.2) [ln(f(c)/f(g))]*.