We investigate through discrete element method (DEM) simulations, the effect of boundary vibrations on the macroscopic friction of shear flows under pressure, in the absence of gravity. The system consists of identical, spherical, frictionless particles confined between two parallel bumpy planes. These planes are constructed by glueing one layer of particles having the same diameter as the flowing particles in a hexagonal closed-packed arrangement. One plane is subjected to harmonic vibrations, while a constant vertical load and constant velocity along the horizontal direction is applied to the other. Previous studies have shown that, above a critical load, such granular flows crystallise under shearing, leading to increased macroscopic friction [1]. The primary objective of this work is to explore whether vibrations can prevent or mitigate crystallisation, thereby reducing the macroscopic friction, inspired by methodologies applied in similar contexts [2]. To address this, we perform the analysis of the spatial distribution of diverse quantities of the granular material as in [3] and conduct a systematic parametric study to identify the conditions under which vibrations affect the flow behaviour. Our results reveal the existence of a frequency window in which the macroscopic friction coefficient decreases consistently across the range of the investigated normal loads. Additionally, we address whether reducing friction necessarily requires the complete destruction of crystalline order. Interestingly, our simulations indicate that even partially crystallised flows can exhibit lower macroscopic friction when subjected to suitable boundary vibrations. These findings provide new insights into the control of granular flow rheology through external mechanical perturbations and may contribute to the design of efficient strategies to modulate friction in granular materials. [1] E. Kurban, D. Vescovi and D. Berzi, “Crystallization in load-controlled shearing flows of monosized spheres”, Soft Matter, 21, 2049-2058, (2025). [2] A. H. Clark, E. E. Brodsky, H. J. Nasrin, S. E. Taylor, “Frictional Weakening of Vibrated Granular Flows ”, Phys. Rev. Lett., 130, 118201 (2023). [3] D. Vescovi, A. S. de Wijn, G. L. Cross and D. Berzi, “Extended kinetic theory applied to pressure-controlled shear flows of frictionless spheres between rigid, bumpy planes”, Soft Matter, 20, 8702-8715, (2024).