Vibrational absorption spectroscopy presents an effective and direct way for molecular detection and identification. In this paper, we propose and demonstrate a simple strategy and structure to amplify molecular detection sensitivity via the example of a monolayer octadecanethiol (ODT). The underlying amplification mechanism operates on both the enhanced surface field in and the coupled-oscillators' energy transfer between the molecules and the cavity underneath. The structure is designed to be simple and free of lithography or patterning with the potential for large-scale uses. It is made of just a quarter wavelength thick dielectric (ZnSe) layer atop a metallic reflecting base. Both angle and polarization dependent reflection spectra reveal signatures of CH2 and CH3 vibrations in theory and experiment. A vibrational signal intensity of 8.54% reached in s-polarization at a large incident angle is comparable to those reported in plasmonic nanostructures with greater sophistications in structure.