  Using the QCD operator product expansion, we derive the real part of the
transverse and longitudinal vector-vector correlation function with the quantum
numbers of the rho and omega mesons to leading order in density and three
momentum (q^2) for energy $\omega^2 --> -\infty $. The operator product
expansion provides, through the Borel transformed energy dispersion relation, a
model independent constraint for the momentum dependence of the vector meson
spectral density in nuclear matter. Existing model calculations for the
dispersion effect of the rho meson, where the vector-meson nucleon scattering
amplitude is obtained by resonance saturation in the s-channel, in general
violate this constraint. We trace this to an inconsistent choice for the form
factor of the $\Delta N\rho$ vertex. With a consistent choice, where both the
form factor and the coupling constant are obtained from the Bonn potential, the
contribution of the $\Delta$ is substantially reduced and we find good
agreement with the constraint equation. We briefly comment on the implications
of our result for attempts to interpret the enhancement of low-mass dileptons
in heavy-ion collisions.
