The long-standing tension between standard Big-Bang Nucleosynthesis (BBN) and the inferred primordial abundance of 7Li suggests that lithium may be a uniquely sensitive diagnostic of small, time-localized departures from the standard early-universe history. In this work we propose a minimal, falsifiable mechanism within the Janus-vacuum program, in which the required structural two-sector conjugation of vacuum contributions is realized through an entangled ground state endowed with an inter-sheet relative phase θ and a metric coherence field χ acting as an effective potential scale for θ. Matter emergence is encoded by an inter-sheet filtering functional Fχ(ω, T, . . .) whose time variability during coherence relaxation produces effective source terms in cosmological number densities. Focusing on BBN, we show how the filtering mechanism can be captured by continuity equations with Janus sources (Sb, Sn) and argue that a weak, time-localized, neutron-rich injection can selectively enhance 7Be conversion and subsequent 7Li burning, thereby reducing the final lithium yield while preserving the successful deuterium and helium predictions. We derive existence criteria, identify a characteristic “window” structure in injection timing and width, and formulate correlated signatures that render the scenario falsifiable under the primaryBBN constraints (D/H and Yp). Lithium thus becomes a direct probe of the Janus-vacuum coherence history: for a given coherence-relaxation model and filtering rule, a viable Janus window either exists or the mechanism is ruled out.