  We study the isospin violating decays of vector charmonia to
$\Lambda\overline{\Sigma}^0$ and its charge conjugate. They are dominated by
the single photon annihilation and can be evaluated reliably with timelike form
factors. We utilize the quark-pair creation model, which is valid for the OZI
suppressed decays, to evaluate the form factors. We obtain the branching
fractions of ${\cal
B}(J/\psi\to\Lambda\overline{\Sigma}^0+c.c.)=(2.4\pm0.4)\times10^{-5}$ and
${\cal
B}(\psi(2S)\to\Lambda\overline{\Sigma}^0+c.c.)=(3.0\pm0.5)\times10^{-6}$, which
are compatible with the measurements by the BESIII collaborations,
respectively. The decay asymmetries are found to be $\alpha_{J/\psi}=0.314$ and
$\alpha_{\psi(2S)}=0.461$, which can be examined at BESIII in the foreseeable
future.
