The effect of the substitution of nearest-neighbor gallium atoms by indium on the frequency of the localized vibrational mode of substitutional nitrogen in the dilute nitride GaNxAs1−x has been studied within first-principles density functional theory, using a supercell approach. The splitting of the highly localized triply degenerate mode into singly and doubly degenerate modes is obtained and compared with available Raman and Fourier transform infrared spectroscopy measurements. The results are in good agreement with the experimental values. The N-In bond formation energy is calculated as 197 meV, and the thermal equilibrium distribution of In-N bonds in Ga1−yInyNxAs1−x at typical annealing temperatures is found to strongly enhance the probability of these bonds, compared to a completely random distribution of In.