Harvested power, extraction efficiency, operating frequency bandwidth and load range of a vibration energy harvester can be improved by using piezoelectric–electromagnetic hybrid vibration energy harvesting technique (PE&EM-VEH). However, the performance of a PE&EM-VEH is significantly influenced by its electrical boundary conditions. In this paper, the problem is investigated by introducing a generalized dimensionless parameter model; based on it, different electrical boundary conditions of a PE&EM-VEH are analyzed and the formulas of their theoretical performances are derived. The relationships between the optimized theoretical performances and the dimensionless parameters are researched by numerical simulations on a PE&EM-VEH with specific parameters; it shows the optimized performances of case (6) (EM-VEH) and case (7) (PE-VEH) are two particular instances of case (9) (PE&EM-VEH) but not the best. Furthermore, experiments on a PE&EM-VEH prototype show that it in case (9) can obtain an optimized power of 2.26mW and an optimized efficiency of 0.41, which increase the power and efficiency by 25.6% and 112% comparing with the average performance of case (6) and case (7). The experimental results demonstrate a good agreement with the theory and experiment; they all validate the potential advantages of an optimized PE&EM-VEH.