We present numerically exact vibrational transitions for trans-HOCO, cis-HOCO, and cis-HOCO− for the quartic force fields of Fortenberry et al. [J. Chem. Phys. 135, 134301 (2011); 135, 214303 (2011)], obtained by means of a computational strategy based on the discrete variable representation. Several adiabatic projection schemes have been employed to characterize the vibrational levels and to study the relevance of the intermode coupling (vibrational mixing). Our results help to clear up a large discrepancy between previously reported vibrational perturbation theory and vibrational configuration interaction predictions for the torsional frequency.