  A family of vertex operators that generalizes those given by Jing for the
Hall-Littlewood symmetric functions is presented. These operators produce
symmetric functions related to the Poincare polynomials referred to as
generalized Kostka polynomials in the same way that Jing's operator produces
symmetric functions related to Kostka-Foulkes polynomials. These operators are
then used to derive commutation relations and new relations involving the
generalized Kostka coefficients. Such relations may be interpreted as
identities in the (GL(n) x C^*)-equivariant K-theory of the nullcone.
