In this work, we report a new generation of sol gel photonic vapor sensor based on water repellent methyl functionalized mesoporous silica thin films for the detection of alcohol vapors. Relative humidity, generally accountable for sensor performance deterioration, was demonstrated as a key parameter to detect and enhance sensor sensitivity concerning ethanol. The ethanol-assisted water vapors capillary condensation into hydrophobic mesopores was observed and investigated as an innovative sensing mechanism. We showed that water condensation was triggered by the surfactant-like behavior of alcohol molecules whose alkyl moieties preferentially interact with the hydrophobic walls and reversibly switch the surface energy from hydrophobic to hydrophilic. Influence of the ethanol and water pressure conditions for which the capillary condensation occurs were studied by in situ-ellipsometry. This study revealed a synergic mechanism of co-adsorption where the minimum ethanol concentration allowing water capillary adsorption decreases when humidity increases. Nanopatterned diffraction gratings films, fabricated via a nanoimprinting process, were demonstrated to be efficient transductor to transform the capillary induced nanoscale variations of refractive index of the porous materials into optical signals easily measurable by conventional camera. Sensing results exhibited an ethanol vapor pressure threshold limit of detection of P/P0 EtOH = 0.07 at 100% relative humidity and typical response and regeneration times are below one minute.