We present a detailed study of vortex motion in a type-II superconductor using the muon spin rotation (mu SR) technique. The vortices were set in motion by an alternating transport current. By adjusting the frequency and amplitude of the driving force so that vortices "visible" to the implanted muons did not cross the sample boundaries, a mu SR line shape was obtained corresponding to almost perfectly ordered vortex motion. We also observed sidebands to the mu SR line shape which correspond to the frequency at which vortices pass the implanted muon, and allow a direct measurement of the vortex velocity. Both these features of the line shape confirm numerical predictions reported in a previous study [see Charalambous , Phys. Rev. B 66, 054506 (2002)].