  Vacuum polarization integrals involve the vector spectral functions which can
be experimentally determined from two sources: (i) e+e- annihilation cross
sections and (ii) hadronic tau decays. Recently results with comparable
precision have become available from CMD-2 on one side, and ALEPH, CLEO and
OPAL on the other. The comparison of the respective spectral functions involves
a correction from isospin-breaking effects which is evaluated. After correction
it is found that the dominant 2pi spectral functions do not agree within
experimental and theoretical uncertainties. Some problems are also found for
the 4pi spectral functions where different experiments do not agree well with
each other. The consequences of these discrepancies for vacuum polarization
calculations are presented, with emphasis on the muon anomalous magnetic
moment. The work includes a complete reevaluation of all exclusive cross
sections, taking into account the most recent data that became available in
particular from the Novosibirsk experiments and applying corrections for the
missing radiative corrections. The values found for the lowest-order hadronic
vacuum polarization contributions are a_mu[had,LO] = (684.7 +- 6.0[exp] +-
3.6[rad])x10^(-10) [e^+e^- -based], and a_mu[had,LO] = (709.0 +- 5.1[exp] +-
1.2[rad] +- 2.8[SU(2)])x10^(-10) [tau-based]. The errors have been separated
according to their sources: experimental, radiative corrections in e+e- data,
and isospin-breaking. We observe deviations of the full SM prediction with the
recent BNL measurement at the 3.0 (e+e-) and 0.9 (tau) sigma level, when adding
experimental and theoretical errors in quadrature.
