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Hard diffractive photo- and electroproduction of heavy vector mesons($J/\psi$ and $\Upsilon$) is evaluated within the leading $\alpha_s\ln{Q^2\over\Lambda_{QCD}^2}$ approximation of QCD. In difference from our earlierwork on that subject, also the production of transversely polarized vectormesons is calculated. Special emphasis is placed on the role of the vectormeson's $q\bar q$ light-cone wave function. In that context, conventionalnon-relativistic quarkonium models and a light-front QCD bound statecalculation are critically examined and confronted with QCD expectations. Ournumerical analysis finds a significant high momentum tail in the latter wavefunctions and a deviation from the expected asymptotic behavior of$\phi_V(z,b=0)\propto z(1-z)$. We then design an interpolation to match thequarkonium models at large inter-quark separations with QCD expectations atsmall distances. We use these results to compare our predictions for theforward differential cross section of $J/\psi$ photo- and electroproductionwith recent experimental results from HERA. In addition, our earlier discussionof $\rho^o$ electroproduction is updated in light of recent experimental andtheoretical enhancements.Comment: 44 pages, LaTeX, 14 figures included using epsf.tex; Final version to  appear in Phys. Rev.

Diffractive heavy quarkonium photoproduction and electroproduction in QCD