We fabricated a vector diffraction grating using a chiral nematic liquid crystal (LC) and a holographically regulated photoalignment film. Periodic defects were observed in the grating LC cell, and the defect position was affected by the temperature of the cell. To characterize the defect position, the director distribution in the cell was estimated on the basis of the elastic continuum theory of chiral nematic LCs. The result clarified that the defect position depended on the helical pitch of the chiral nematic LC. We also investigated diffraction properties of the grating LC cell. The diffraction efficiency and polarization conversion property could be controlled by the temperature. The experimental results were well explained theoretically by considering the temperature dependence of the optical anisotropy and helical pitch of the chiral nematic LC.