Vibration energy harvesting can be used as a sustainable power source for various applications. Usually, the generators are designed as devices with a single degree of freedom (SDoF) along the direction of the driving motion. In this research, harvesting from multi-directional (translational) motion sources will be investigated. Three strategies are assessed: a reference SDoF generator, a SDoF generator using an orientation strategy, and a Multi Degree of Freedom (MDoF) system. This led to the development of a design metric by which any 2D design problem can be described by two dimensionless parameters: the relative strength of vibrations, p_v, and the relative dimension of the design space, p_l. It was shown that the relative power density (RPD) of a 2DoF system compared to a reference SDoF system only depends on the product p^∗=p_vp_l, and has a maximum of 1.185 for p^∗=1. The application of powering a hearing aid is investigated as a case study. It was found that the vibrations in the area of the human head while walking can be represented by a two-directional vibration source with p_v=0.55. Three different design spaces are assessed for a miniaturized generator and three different optimal embodiments are found. For one of the considered situations where p^∗=1.1, a 2DoF system was found to have a 16% higher power output compared to a SDoF reference. The aim of future work will be the validation of the developed metric.