We present a scalable method for producing monodisperse microscale emulsions in a container holding two stably stratified layers of immiscible liquids by applying horizontal vibration. Our experiments and theoretical modelling show that the critical non-dimensional acceleration for regular droplet formation is governed by a shear-dominated breakup mechanism, which scales as $N^{-1/2} omega^{*3/2}$, where $N$ is the viscosity ratio and $omega^{*}$ is the frequency of forcing on the viscous-capillary scale. The droplet diameter can be easily controlled by varying the forcing parameters, thus demonstrating this vibrational configuration as a scalable alternative to microfluidics.