<p>The outstanding mechanical properties of V-alloyed medium Mn steels have received significant attention in recent years. The quantitative effect of Mn content on either stoichiometry or the solubility product of VC_<i>x</i> (vanadium carbides) in austenite remains unclear. The thermodynamic equilibrium between nonstoichiometric vanadium carbide and Mn-added austenite using a two-sublattice model have been investigated in this research, in which quantitative experiments have been employed to derive the interaction parameter <i>L</i>fcc Mn:C,Va for the calculations. The effect of Mn on the solubility products of VC_0.87 and VC_0.75 in austenite was calculated and compared with VC. The results indicate that Mn increases the solubility products of VC_0.87 and VC_0.75, although this effect diminishes as the carbon vacancy concentration in vanadium carbide decreases. Based on fixed compositions in the Fe-Mn-V-C system, the effects of Mn on the C/V atomic ratio (<i>x</i>) and the solubility product of VC_<i>x</i> in austenite were calculated. The results revealed that an increase in Mn reduced the precipitated mass and C/V atomic ratio, while increasing the logarithmic solubility product of VC_<i>x</i>. In addition, the calculation results were verified through extraction experiments. The quantitative solubility product of vanadium carbides involving stoichiometry provides a crucial reference for material development of the Fe-Mn-V-C systems.</p>