  We experimentally and numerically study the collapse dynamics of a quantum
vortex in a two-dimensional atomic superfluid following a fast interaction ramp
from repulsion to attraction. We find the conditions and time scales for a
superfluid vortex to radially converge into a quasi-stationary density profile,
demonstrating the first spontaneous formation of a vortex soliton in an atomic
Bose gas. We record an emergent universal dynamics of an azimuthal modulational
instability, which amplifies initial density perturbations and leads to the
eventual splitting of a vortex soliton or direct fragmentation of a superfluid
into disordered, but roughly circular arrays of Townes soliton-like
wavepackets. Our study sets the stage for exploring universal
out-of-equilibrium dynamics of vortex quantum matter quenched to attractive
interactions.
