  We examine the total cross section of virtual photons on protons,
$\sigma_{\gamma^* p}(W^2,Q^2)$, at low $x \cong Q^2/W^2 \ll 1$ and its
connection with ``elastic'' diffractive production $\gamma^*_{T,L}p \to
X^{J=1}_{T,L} p$ in the two-gluon exchange dynamics for the virtual forward
Compton scattering amplitude. Solely based on the generic structure of
two-gluon exchange, we establish that the cross section is described by the
(imaginary part of the) amplitude for forward scattering of $q \bar q$ vector
states, $(q \bar q)^{J=1}_{T,L} p \to (q \bar q)^ {J=1}_{T,L} p$. The
generalized vector dominance/color dipole picture (GVD/CDP) is accordingly
established to only rest on the two-gluon-exchange generic structure. This is
explicitly seen by the sum rules that allow one to directly relate the total
cross section to the cross section for elastic diffractive forward production,
$\gamma^*_{T,L} p\to (q \bar q)^{J=1}_{T,L} p$, of vector states.
