  If Nature has chosen the vacuum oscillation solution to the Solar neutrino
puzzle, a key theoretical challenge is to understand the extreme smallness of
the $\Delta m^2_{\nu_e-\nu_X}$ ($\sim 10^{-10} eV^2$) required for the purpose.
We find that in a class of models such as [SU(3)]^3 or its parent group E_6,
which contain one sterile neutrino, $\nu_{is}$ for each family, the $\Delta
m^2_{\nu_i-\nu_{is}}$ is proportional to the cube of the lepton Yukawa
coupling. Therefore fitting the atmospheric neutrino data then predicts the
$\nu_e-\nu_{es}$ mass difference square to be $\sim (\frac{m_e}{m_{\mu}})^3
\Delta m^2_{atmos}$, where the atmospheric neutrino data is assumed to be
solved via the $\nu_{\mu}-\nu_{\mu s}$ oscillation. This provides a natural
explanation of the vacuum oscillation solution to the solar neutrino problem.
