  We investigate the problem of the successive refinement for Wyner-Ziv coding
with degraded side information and obtain a complete characterization of the
rate region for the quadratic vector Gaussian case. The achievability part is
based on the evaluation of the Tian-Diggavi inner bound that involves Gaussian
auxiliary random vectors. For the converse part, a matching outer bound is
obtained with the aid of a new extremal inequality. Herein, the proof of this
extremal inequality depends on the integration of the monotone path argument
and the doubling trick as well as information-estimation relations.
