This study proposes and investigates a novel nonlinear vibration energy harvesting (VEH) configuration characterized by tunable stiffness, damping, and inertia. The system integrates adjustable-inertia components and X-shaped support structures. A key innovation lies in the inertia modulation mechanism, which enhances mode coupling between translational and rotational motions—an effect that significantly improves energy harvesting performance. The X-shaped supports introduce mild nonlinearities in both stiffness and damping, contributing to a broadened operational bandwidth. By synergistically combining the mode coupling effect with the nonlinear mechanical properties of the support structure, the proposed design achieves superior energy harvesting efficiency across both low-frequency and wide-spectrum vibration inputs. The resulting two-degree-of-freedom (2DOF) nonlinear VEH architecture demonstrates the potential to outperform both its linear 2DOF counterpart and existing nonlinear systems. These findings offer a promising direction for the passive structural optimization of VEH systems, particularly in enhancing performance within the low-frequency regime.