Mechanical vibrations are known to affect frictional sliding and the associated stick-slip patterns causing some- times a drastic reduction of the friction force. This issue is relevant for applications in nanotribology and to un- derstand earthquake triggering by small dynamic pertur- bations. We study the dynamics of repulsive parti- cles confined between a horizontally driven top plate and a vertically oscillating bottom plate. Our numerical results show a suppression of the high dissipative stick-slip regime in a well defined range of frequencies that depends on the vibrating amplitude, the normal applied load, the system inertia and the damping constant. We propose a theoreti- cal explanation of the numerical results and derive a phase diagram indicating the region of parameter space where friction is suppressed.