  Although the meson decay amplitude described by a two-point function may be
regarded as one of the simplest possible physical observable, it is interesting
that this apparently simple amplitude bears abundant fundamental informations
on QCD vacuum dynamics and chiral symmetry. The light-front zero-mode issue of
the vector meson decay constant $f_V$ is in this respect highly non-trivial and
deserves careful analyses. We discuss the zero-mode issue in the light-front
quark model (LFQM) prediction of $f_V$ from the perspective of the vacuum
fluctuation consistent with the chiral symmetry of QCD. We extend the exactly
solvable manifestly covariant Bethe-Salpeter model calculation to the more
phenomenologically accessible realistic LFQM and present a self-consistent
covariant description of $f_V$ analyzing the twist-2 and twist-3
quark-antiquark distribution amplitudes with even chirality.
