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Using dispersive methods, we study the $B \to \gamma^*$ form factors underlying decay $B^- \ell^- \bar{\nu}_\ell \ell^{\prime-} \ell^{\prime+}$. We discuss ambiguity that arises from a separation of full \ell^{\prime+}$ amplitude into hadronic tensor and final-state-radiation piece, including effects nonvanishing lepton masses. For eligibility treatment, propose decomposition part leads to four are free kinematic singularities. By establishing set dispersion relations, then relate well-known V$, $V=\omega(782),\rho(770)$, analogues. combination series expansion in conformal variable vector-meson-dominance ansatz parameterize factors, infer values associated unknown parameters available input on V$. The phenomenological application our formalism includes determination branching ratios forward-backward asymmetries process for two lightest leptons.