  Taking into account the first localization corrections in the
electron-impurity self-energy we study the effect of non-magnetic impurities on
unconventional superconductivity (SC) mediated by small-q electron-phonon
scattering. We show that when van Hove singularities are close to the Fermi
level making the electronic system anisotropic as in the high-$T_c$ oxides,
both the d-wave and s-wave states are sensitive to non-magnetic impurities and
beyond a critical impurity concentration SC disappears in {\it both gap
symmetry channels}. Impurity doping may induce a first order transition from
d-wave to s-wave SC, but no saturation of the impuruty effect is reported due
to the intrinsically anisotropic character of the localization corrections in
this context.
