The laws of physics have never specifically ruled out the possibilty of a device that could take the energy of the vacuum and convert it to useful work. Instead thermodynamics outlines the conditions that must be neccessary for a heat engine (which a ZPE device must be) to function in the environment of the 4 dimensional universe.
The first requirement is the existence of a heat source and a heat sink, a difference in temperature that is bridged by the engine. In this respect the vacuum must be considered as both the source and sink, which might deliver a fatal blow to our designs.
The nature of the vacuum is different than any thermodynamic media we are familiar with in that we cannot assign a temperature to the vacuum that is universal. We might say that the temperature of the vacuum is 4 kelvins from the background big bang radiation, but this not the true temperature of the vacuum, but of the Universe on a macroscopic scale.
The temperature of the vacuum is scale dependent in other words, the smaller the scale the higher the temperature. This is otherwise known as the Uncertainty Principle. The smaller the space we try to squeeze an electron the higher its momentum.
Our engine then must bridge the gap between two energy scales, in other words two cavities of different dimensions, even better the cavities should be harmonic multiples of each other.
Other considerations of scale should tell us the dimensions of these cavities should be extremely small so that the higher energy modes of the vacuum can exploited. There is also a maximum dimension of the cavity that is determined by the ambient temperature. The energy density of the enviroment must be less than that of the device for the energy produced to actually show up as useful work. This is why the large scale devices with spinning wheels and diodes produced by many inventors can be written off as inoperable. For the temperatures that we experience, this dimension is in the region of wavelengths of the far infrared, which makes our engine buildable by photolithographic means. We will never actually be able to see our ZPE engine except under a microscope, and when we do there will be billions of them working in parallel,producing electrical current quietly and without end.