Novel poly(vinylidene fluoride) (PVDF)/poly(glycidyl methacrylate) (PGMA) blend membranes were successfully fabricated via the nonsolvent-induced phase separation (NIPS) process. Theoretical prediction using the Schneier equation and comprehensive characterization (XRD, FTIR, DSC, FESEM) established that the system exhibits partial miscibility with a critical phase separation threshold around 37 vol % PGMA. Below this threshold, good polymer miscibility was evidenced by favorable thermodynamic parameters and spectroscopic shifts. Beyond it, clear macroscopic phase separation occurred, influencing crystal uniformity and morphology. Crucially, the NIPS process strongly promoted the crystallization of PVDF into the polar, electroactive β-phase. The β-phase content was significantly enhanced from 17% in neat PVDF powder to a maximum of 70% in the optimized blend, despite the overall degree of crystallinity remaining relatively low (26–31%). Moreover, melting the NIPS-formed blends further enhanced the β-phase content at low-to-mid PGMA concentrations. Beyond the crystalline phase modulation, PGMA incorporation effectively modulated the membrane microstructure, significantly enhancing both porosity and surface hydrophilicity. The demonstrated ability to tune the microstructure and polar phase formation through simple blend composition makes these PVDF/PGMA membranes highly promising candidates for advanced functional and biomedical applications.