High resolution IR laser direct absorption spectra in a slit jet are presented and analyzed for nitrogen ((NN)-N-15-N-14-HF, (NN)-N-15-N-15-HF), and deuterium ((NN)-N-14-N-14-DF) substituted N2HF isotopomers. Both (NN)-N-14-N-15-HF and (NN)-N-15-N-14-HF isomers are observed, indicating a sufficiently deep minimum in the hydrogen bonding potential energy surface to quench internal rotation of the N-2. The vibrationally averaged stretching potentials for each substituted species are recovered from rotational Rydberg-Klein-Rees (RKR) analysis. Features of the one-dimensional (1D) potential surface such as hydrogen bond length (R(H-bond)), harmonic force constant (k(sigma)), and well depth (D-e) are then tested for isotopic invariance by direct comparison of the different isotopomers. Agreement among the various N substituted species for HF based complexes for either upsilon(HF)=0 or 1 is excellent, and provides effective 1D potentials for the stretching coordinate between 3.39 and 3.75 Angstrom. There is a 43 cm(-1) (similar to 10%) strengthening of the hydrogen bond upon HF vibrational excitation, as quantitatively reflected in the experimental redshifts and the shape of the RKR potentials for upsilon(HF)=0 and 1. The hydrogen bond is further strengthened by D/H isotopic substitution; this is a result of reduced vibrational averaging over DF vs HF bending motion, yielding a more linear, and hence stronger, hydrogen bond geometry. In contrast to the nearly apparatus-limited linewidths (Delta upsilon(prediss)similar to 7 MHz) observed for each of the N2HF isotopomers, the N2DF complexes yield significantly broadened lines with 73+/-9 MHz homogeneous linewidths due to vibrational predissociation. This tenfold increase in predissociation rates upon deuteration is in contrast to previous measurements in other HF/DF containing complexes, and indicates the importance of a near resonant vibrational channel to form N-2(upsilon=1)+DF(upsilon=0). The energetic accessibility of this V-->V channel would suggest an upper limit on the N2DF binding energy of D-0 less than or equal to 547 cm(-1), which is also consistent with upper limits on D-0 from the rotational RKR analysis.