  Specializations of Schur functions are exploited to define and evaluate the
Schur functions s_\lambda[\alpha X] and plethysms s_\lambda[\alpha s_\nu(X))]
for any \alpha - integer, real or complex. Plethysms are then used to define
pairs of mutually inverse infinite series of Schur functions, M_\pi and L_\pi,
specified by arbitrary partitions \pi. These are used in turn to define and
provide generating functions for formal characters, s_\lambda^{(\pi)}, of
certain groups H_\pi, thereby extending known results for orthogonal and
symplectic group characters. Each of these formal characters is then given a
vertex operator realization, first in terms of the series M=M_{(0)} and various
L_\sigma^\perp dual to L_\sigma, and then more explicitly in exponential form.
Finally the replicated form of such vertex operators are written down.
