  We discuss in detail the flavour structure of the supersymmetric SO(10) grand
unified models with the three traditional 16-dimensional matter spinors mixed
with a set of extra 10-dimensional vector multiplets which can provide the
desired sensitivity of the SM matter spectrum to the GUT symmetry breakdown at
the renormalizable level. We put the qualitative argument that a successful fit
of the quark and lepton data requires an active participation of more than a
single vector matter multiplet on a firm, quantitative ground. We find that the
strict no-go obtained for the fits of the charged sector observables in case of
a single active matter 10 is relaxed if a second vector multiplet is added to
the matter sector and excellent, though non-trivial, fits can be devised.
Exploiting the unique calculable part of the neutrino mass matrix governed by
the SU(2)_L triplet in the 54-dimensional Higgs multiplet, a pair of genuine
predictions of the current setting is identified: a non-zero value of the
leptonic 1-3 mixing close to the current 90% C.L. limit and a small leptonic
Dirac CP phase are strongly preferred by all solutions with the global-fit
chi^2-values below 50.
