Rotationally resolved infrared spectra are reported for the HCN dimer, grown and solvated in liquid helium droplets. This is the first study for which two different vibrational modes within the same liquid helium solvated molecule have been observed, namely those associated with the "free" and the "hydrogen-bonded" C-H stretching vibrations. Comparing the line broadening in these two bands, we conclude that the helium solvent plays an important role in the vibrational relaxation dynamics of the dimer. The rotational constants obtained from these spectra indicate that the dimer rotates more slowly in the liquid than in the gas phase.