  Microscopic theory of the type of Efetov's supermatrix sigma-model is
constructed for the low-lying electron states in a mixed superconductive-normal
system with disorder. The developed technique is used for the study of the
localized states in the core of a vortex in a moderately clean superconductor
(1/\Delta << \tau << 1/\omega_0 = E_F/\Delta^2). At sufficiently low energies E
<< \omega_{Th}, the energy level statistics is described by the
"zero-dimensional" limit of this supermatrix theory, with the effective
"Thouless energy" \omega_{Th} \sim (\omega_0/\tau)^{1/2}. Within this energy
range the result for the density of states is equivalent to that obtained
within Altland-Zirnbauer random matrix model of class C. Nonzero modes of the
sigma-model increase the mean interlevel distance \omega_0 by the relative
amount of the order of [2\ln(1/\omega_0\tau)]^{-1}.
