The study of magnetic quantum oscillations in the superconducting state is of fundamental importance for understanding the nature of superconductivity under high magnetic fields. However, although studied for more than three decades, this phenomenon poses several basic questions that still defy satisfactory answers. A key controversial issue concerns the additional damping observed in the vortex state of many strong type-II superconductors. Here, we show results of μSR, dHvA, and superconducting quantum interference device magnetization measurements on borocarbide superconductors, initially aimed at investigating the “phase-smearing” effect due to inhomogeneous field broadening. It is found, however, that a sharp drop observed in the dHvA amplitude just below Hc2 is correlated with enhanced disorder of the vortex lattice in the peak-effect region, where the phase-smearing effect is negligible. It is concluded that quasiparticle scattering by the pair potential is significantly enhanced due to vortex-lattice disorder, thus generating additional damping in the dHvA amplitude.