  The differential cross section $d\sigma/dq^2$ of diffractive
electroproduction of heavy quarkonia on protons is a sensitive study tool for
the interaction dynamics within the dipole representation. Knowledge of the
transverse momentum transfer $\vec q$ provides a unique opportunity to identify
the reaction plane, due to a strong correlation between the directions of $\vec
q$ and impact parameter $\vec b$. On top of that, the elastic dipole-proton
amplitude is subject to a strong correlation between $\vec b$ and dipole
orientation $\vec r$. Most of models for $b$-dependent dipole cross section
either completely miss this information, or make unjustified assumptions. We
perform calculations basing on a realistic model for $\vec r$-$\vec b$
correlation, which significantly affect the $q$-dependence of the cross
section, in particular the ratio of $\psi^{\,\prime}(2S)$ to $J/\psi$ yields.
We rely on realistic potential models for the heavy quarkonium wave function,
and the Lorentz-boosted Schr\"odinger equation. Good agreement with data on
$q$-dependent diffractive electroproduction of heavy quarkonia is achieved.
