To summarize, we have demonstrated that an in-situ membrane-based GO reduction method using VC aqueous solution can largely retain the sheet morphology of rGO via laminar confinement. DFT simulations revealed that the mechanism toward the reduction of the epoxy and hydroxyl groups on GO by VC proceeds via low-lying transition structures and has a strong thermodynamic drive. Thus, these reductions are both kinetically and thermodynamically favorable. The VG-M material from our method exhibited much better supercapacitor performance than VG-S from conventional method in a free environment. Generally, for VG-M, a specific capacitance of 174 F/g can be obtained at 1 A/g and 75.9% of retention can be achieved at 40 A/g, which is about 9 times better than VG-S. The significant performance superiority of VG-M can be attributed to the retained sheet morphology that provides more available sites for electrolyte ion storage. Compared to VG-S that is reduced in a free environment, VG-M is reduced in a highly confined environment, where the confinement counters the increased in-plane strain from reduction and thus maintains its original morphology. This study not only proposes a simple and promising process for producing a high-performance supercapacitor material but also provides a new insight for tuning the morphology of 2D materials.