The dHvA oscillations in crystals of LuNi₂B₂C are known to persist deep inside the vortex state, across the region of the peak effect located at the edge of the irreversibility field normal phase boundary. This makes the identification of H_c2(T) ambiguous in magnetization hysteresis loops at temperatures at which the dHvA signal is prominent. One can overcome this difficulty if H_c2(T) value is ascertained from the measurement of isothermal quadrupole moment (Q) vs.field values using a vibrating sample magnetometer. In these crystals, one can witness dHvA oscillations up to about 8K and we show that Q(T) data in LuNi₂B₂C can be used to construct the vortex phase diagram over the entire (H,T) range.