  We present an inelastic neutron scattering study of phonon lineshapes in the
vortex state of the type-II superconductor YNi$_2$B$_2$C. In a previous study
[Phys. Rev. Lett. \textbf{101}, 237002 (2008)] it was shown that certain
phonons exhibit a clear signature of the superconducting gap $2\Delta$ on
entering the superconducting state. Our interest was to find out whether or not
the lineshape of such phonons reflects the inhomogeneous nature of the vortex
state induced by a magnetic field smaller than the upper critical field
$B_{c2}$ .We found that this is indeed the case because the observed phonon
lineshapes can be well described by a model considering the phonon as a local
probe of the spatial variation of the superconducting gap. We found that even
at $B=3\,\rm{T}$, where the inter-vortex distance is less than $300\,$\AA, the
phonon lineshape still shows evidence for a variation of the gap.

Superconductivity