Building on previous studies involving coupled cluster quartic force fields for the description of spectroscopic constants and vibrational frequencies of astronomically relevant molecules, this work applies the same techniques to the elucidation of such properties for the bent 1 (3)A' state of HCN and the isoelectronic 1 (3)A' HCO+. Core correlation is treated both by explicit means and as a correction. Each approach gives closely comparable spectroscopic constants and vibrational frequencies once more, indicating that the composite method is a viable and less costly alternative. We are providing fundamental vibrational frequencies for these systems where agreement with experiment in previous studies has been within 4 cm(-1) or better. Frequencies for the first overtones and combination bands as well as various spectroscopic constants are also reported.