Phase separation has been indicated as the best approach to maximize energy production from sugarcane vinasse via biodigestion. However, process specificities, such as non-removing sulfate during fermentation, may negatively impact methanogenesis. This study assessed methane evolution in batch systems fed with different vinasse types, i.e., raw (RVin), sulfate-rich fermented (FVin+SO4) and sulfate-poor fermented (FVin-SO4) vinasses, detailing the metabolic shifts required to supply methanogens with acetate in each case. The incomplete oxidation of glycerol and lactate by sulfate reducers provided acetate in a shortcut-like pathway, promptly boosting methane evolution from RVin and FVin+SO4. However, the stimulus to methanogenesis takes its toll by impairing the final methane yield and biogas quality (up to 5.6% of sulfide). Acetate provision exclusively by the syntrophic oxidation of butyrate and phenol-derived intermediates slowed down methanogenesis. Controlling vinasse fermentation towards enhanced sulfate removal and acetate production will be imperative for maximizing bioenergy recovery in methanogenic systems.