This paper presents a unified model of piezoelectric vibration energy harvesters through the use of a generalized electrical impedance that represents various energy harvesting interfaces, providing a universal platform for the analysis and discussion of energy harvesters. The unified model is based on the equivalent circuit analysis that utilizes the impedance electromechanical analogy to convert the system into the electrical domain entirely, where the model is formulated and analyzed. Firstly, the common behaviors of energy harvesters under this unified model are discussed, and the concept of power limit is discussed. The power limit represents the maximum possible power that could be harvested by an energy harvester regardless of the type of the circuit interface. The condition to reach this power limit is obtained by applying the impedance matching technique. Secondly, three representative energy harvesting interfaces, i.e., resistive (REH), standard (SEH), and synchronized switch harvesting on inductor (SSHI), are discussed separately, including their corresponding forms of the generalized electrical impedance and associated system behaviors. As an important contribution, a clear explanation of the system behavior is offered through an impedance plot that graphically illustrates the relationship between the system tuning and the harvested power. Thirdly, the effect of the system electrometrical coupling on power behaviors is discussed. As another important contribution, this paper derives and presents the analytical expressions of the critical coupling of the interfaces, which is the minimum coupling required to reach the power limit and also the parameter used to define the coupling state, i.e., weakly, critically, or strongly, of a system. In particular, the analytical expressions for the SEH and SSHI interfaces are presented for the first time in the research community. Lastly, the system behaviors and critical coupling of the three energy harvesting interfaces are compared and discussed. The SSHI interface has the lowest critical coupling, which explains its superior power harvesting capability for weakly coupled systems.