The vibrational dynamics of formic acid dimer is quite regular at low fundamental excitation frequencies, whereas it evolves into a complex and irregular vibrational signature in the OH stretching region. This is evidenced by the first Raman investigation of the jet-cooled formic acid dimer and its three deuterated isotopomers. Subtle isotope effects in the inter-monomer stretching mode, which is directly observed for the first time at 194 cm(-1), find an interpretation based on hydrogen bond weakening due to quantum delocalization of the protons. The reported high-frequency jet spectra should provide essential experimental stepping stones towards a more complete understanding of this planar prototype for strong double hydrogen bonding.