A selected ion flow drift tube (SIFDT) has been used to measure vibrational quenching rate constants of HCN+ and DCN+ in collisions with He from 0.05 to 0.35 eV mean relative kinetic energy. The measured quenching rate constants fit linear Landau-Teller (L-T) plots in the 0.20 to 0.35 eV energy range, equivalent to 1540 to 2700 K effective temperature range. These are the first L-T plots for polyatomic ion vibrational quenching. The L-T plots imply large vibrational excitation in the CH (CD) fundamental stretching mode by electron impact ionization of HCN in the threshold to similar to 40 eV energy range. The L-T plots also imply that vibrational quenching of the CH (CD) fundamental stretching mode by He is either directly to the ground vibrational state, or possibly to the low-frequency bending mode, but not via the CN stretching mode. There is a large HNC+ ion (45%+/- 5%) production by electron impact ionization of HCN. This is in contrast with HCN photoionization, which yields only HCN+.