We present a new phase mask coronagraph consisting in an optical vortex induced by a space-variant surface relief subwavelength grating. Phase mask coronagraphy is a recent technique aiming at accommodating both high dynamic and high angular resolution imaging of faint sources around bright objects such as exoplanets orbiting their parent stars or host galaxies of active galactic nuclei. Subwavelength gratings are known to be artificially birefringent. Their unique dispersive characteristics can be controlled through the grating geometry in order to synthesize achromatic phase shifters. We show that implementing them in a ring-shaped way produces a fully symmetric and achromatic coronagraph without any gap or "dead zone." The practical manufacturing of the device is also discussed.