Using a molecular beam opthothermal spectrometer, we have recorded the sub-Doppler ir rovibrational spectra of the CH stretching vibrations of the C-3 upsilon Symmetric-top van der Waals molecules HCCCN-BF3 (upsilon(1) fundamental) and HCN-BF3 (upsilon(1) fundamental and first overtone). At first sight, all spectra appear regular, showing no signs of extensive (local) perturbations as might have been expected from the similarities in structure between HCN-BF3 and the stable molecule HCCCF3, of which the 2 upsilon(1) spectrum in particular presents a clear example of intermediate case intramolecular vibrational energy relaxation. However, closer inspection reveals that the HCCCN-BF3 spectrum shows evidence of an isotope dependent vibrational predissociation rate, the B-11 complex dissociating at least twice as fast as its B-10 counterpart. The complexation induced red shift equals 2.70 cm(-1), and is in the same range as observed for the majority of the HCCCN and HCN complexes studied to date. In contrast, the HCN-BF3 spectra show instrument limited linewidths, therewith providing only a lower limit to the lifetime, but the complexation induced frequency shifts are clearly anomalous : the fundamental spectrum is red shifted by only 0.15 cm(-1) while the overtone spectrum is blue shifted by 0.66 cm(-1). These observations are rationalized by invoking long-range (anharmonic) vibrational interactions in both the HCCCN-BF3 and the HCN-BF3 molecule.