Isobaric Vapor–Liquid Equilibrium for the N -Methyl-2-pyrrolidone + 1,4-Butanediol Binary System at 40.0, 20.0, and 10.0 kPa

N-Methyl-2-pyrrolidone (NMP) is a critical solvent in diverse industrial applications, particularly in the microelectronics and lithium-ion battery sectors, where high purity is paramount. A promising purification strategy involves the reactive conversion of its persistent impurity, γ-butyrolactone (GBL), into 1,4-butanediol (BDO). To provide essential design data for the subsequent distillation-based separation of NMP + BDO mixture, isobaric vapor–liquid equilibrium (VLE) data were experimentally measured at reduced pressures of 40.0, 20.0, and 10.0 kPa using a modified Othmer still. The experimental VLE data underwent rigorous thermodynamic consistency verification using both the Fredenslund’s test and Van Ness’s point-to-point test. Subsequently, these data were successfully correlated with the Wilson, NRTL, and UNIQUAC models. All three models demonstrated an excellent fit to the experimental data. Furthermore, the thermodynamic consistency of the regressed binary interaction parameters was confirmed via a graphical analysis of the Gibbs energy of mixing. These comprehensive VLE data and validated model parameters are crucial for the accurate design, simulation, and optimization of the NMP purification process.