  We discuss results on the diffractive production of the vector mesons rho,
phi and omega reported by the H1 and ZEUS collaborations at HERA. Unique to
such studies is the experimental accessibility to the polarization of the
vector mesons and hence to the spin-density matrix elements arising in
vacuum-exchange processes. We emphasize the relation between the observed
dependence on momentum transfer and the polarization state of the vector meson.
The diffractive nature of the production mechanism is investigated via
extraction of the Pomeron trajectory at high Q^2. Flavor symmetry is observed
in the phi/rho ratios in the same region of momentum transfer where the
power-law scaling becomes similar. The multivariable helicity analyses impose
stringent constraints on models for the vacuum-exchange production mechanism.
Semi-exclusive photoproduction of transverse rho and phi mesons at momentum
transfers far exceeding their mass scale exhibit a hard scaling behavior which
appears to violate the QCD helicity selection rules in a two-gluon exchange
model.
