  The Enhanced Resolution Imager and Spectrograph (ERIS) is the new
near-infrared instrument at the VLT-UT4. ERIS replaces and extends the
observational capabilities formerly provided by SINFONI and NACO: integral
field spectroscopy at 1 - 2.5 $\mu$m, imaging at 1 - 5 $\mu$m with several
options for high-contrast imaging, and long-slit spectroscopy. In particular, a
vortex coronagraph is now available for high contrast observations at L and M
band. It is implemented using annular groove (or vortex) phase masks (one for
each of the L and M bands) in a focal plane, and a Lyot stop in a downstream
pupil plane. The vortex coronagraph has a discovery space starting already at
$\sim$1$\lambda/D$, and works well in broadbands. However, to reach its optimal
performance, it is critical to correct for slow pointing errors onto the vortex
phase mask, which mandates a dedicated pointing control strategy. To do so, a
control loop based on the QACITS algorithm has been developed and commissioned
for ERIS. Good pointing stability is now regularly achieved with errors between
0.01 and 0.02 $\lambda/D$ and a correction rate of 0.2 Hz. In this
contribution, we first review the design of the ERIS vortex coronagraph. We
then detail the implementation of the QACITS algorithm describing the entire
observing sequence, including the calibration steps, the initial centering, and
the stabilization during the observing template. We then discuss performance
based on commissioning data in terms of pointing accuracy and stability.
Finally, we present post-processed contrast curves obtained during
commissioning and compare them with NACO vortex data, showing a significant
improvement of about 1 mag at all separations.
