  The impurity problems within vortex cores of two-dimensional s-wave and
chiral p-wave superconductors are studied numerically in the framework of the
quasiclassical theory of superconductivity and self-consistent Born
approximation under a trial form of the pair potential. The dispersion and
impurity scattering rate (the inverse of the relaxation time) of the Andreev
bound state localized in vortex cores are deduced from the angular-resoloved
local density of states. The energy dependence of the impurity scattering rates
depends on the pairing symmetry; particularly, in the chiral p-wave vortex core
where chirality and vorticity have opposite sign and hence the total angular
momentum is zero, the impurities are ineffective and the scattering rate is
vanishingly small. Owing to the cancellation of angular momentum between
chirality and vorticity, the chiral p-wave vortex core is similar to locally
realized s-wave region and therefore non-magnetic impurity is harmless as a
consequence of Anderson's theorem. The results of the present study confirm the
previous results of analytical study (J. Phys. Soc. Jpn. {\bf 69} (2000) 3378)
in the Born limit.
