We theoretically and experimentally investigate the effect of imperfect vector symmetry on radially polarized beams focused by an aplanatic solid immersion lens at a numerical aperture of 3.3. We experimentally achieve circularly symmetric focused spot with a full-width-half-maximum of ~λ0/5.7 at λ0 = 1,310 nm, free-space wavelength. The tight spatial confinement and overall circular symmetry of the focused radially polarized beam are found to be sensitive to perturbations of its cylindrical polarization symmetry. The addition of a liquid crystal based variable retarder to the optical path can effectively ensure the vector symmetry and achieve circularly symmetric focused spots at such high numerical aperture conditions.