Onset of the KHI in run dg3c4_2048 in a region at the disk upper layers, where the instability first occurs. Top: snapshots at different times (in units of the orbital period at 5.5 au) showing the deviation of the radial Richardson number (Eq. (1)) from its critical value of 1/4 required to trigger the KHI. Black regions correspond to Ri > 1/4. Bottom: same snapshot showing the azimuthal vorticity, highlighting the formation of eddies. The rightmost panels show the same quantities > 200 orbits after VSI saturation.