  Gap-opening planets can generate dust-trapping vortices that may explain some
of the latest discoveries of high-contrast crescent-shaped dust asymmetries in
transition discs. While planet-induced vortices were previously thought to have
concentrated shapes, recent computational work has shown that these features
naturally become much more elongated in the gas when simulations account for
the relatively long timescale over which planets accrete their mass. In this
work, we conduct two-fluid hydrodynamical simulations of vortices induced by
slowly-growing Jupiter-mass planets in discs with very low viscosity ($\alpha =
3 \times 10^{-5}$). We simulate the dust dynamics for four particle sizes
spanning 0.3 mm to 1 cm in order to produce synthetic ALMA images. In our
simulations, we find that an elongated vortex still traps dust, but not
directly at its center. With a flatter pressure bump and disruptions from the
planet's overlapping spiral density waves, the dust instead circulates around
the vortex. This motion (1) typically carries the peak off-center, (2) spreads
the dust out over a wider azimuthal extent $\geq 180^{\circ}$, (3) skews the
azimuthal profile towards the front of the vortex, and (4) can also create
double peaks in newly-formed vortices. In particular, we expect that the most
defining observational signature, a peak offset of more than $30^{\circ}$,
should be detectable $>30\%$ of the time in observations with a beam diameter
of at most the planet's separation from its star.
