A bstract We investigate various search strategies for light vector boson X in O (10) MeV mass range using J/ Ψ associated channels at BESIII and Belle II: (i) J/ Ψ → η c X with 10 10 J/ Ψs at BESIII, (ii) J/ Ψ( η c + X ) + ℓ ℓ ¯ production at Belle II, and (iii) J/ Ψ + X with the displaced vertex in X → e + e − decay are analyzed and the future sensitivities at Belle II with 50 ab − 1 luminosity are comprehensively studied. By requiring the displaced vertex to be within the beam pipe, the third method results in nearly background-free analysis, and the vector boson-electron coupling and the vector boson mass can be probed in the unprecedented range, 10 − 4 ≤ | ε e | ≤ 10 − 3 and 9 MeV ≤ m X ≤ 100MeV with 50 ab − 1 at Belle II. This covers the favored signal region of 8 Be * anomaly recently reported by Atomki experiment with m X ≃ 17 MeV.