High-resolution resonant inelastic X-ray scattering (RIXS) provides direct access to nuclear dynamics in molecular core-excited states through the vibrational structure of the spectra. Here, we investigate vibrationally resolved RIXS at the nitrogen K edge of ammonia by combining high-resolution measurements with ab initio calculations and quantum-dynamical simulations. Potential energy surfaces of the lowest core-excited states are computed using the ADC(2)-x and RASPT2 methods and employed in a multimode treatment including all six vibrational normal modes of the molecule. For the bound doubly degenerate 2e core-excited state, the calculated RIXS spectra are in good agreement with experiment and reveal dominant contributions from the umbrella and N–H stretching modes. The simulations explain the excitation-energy dependence of the vibrational structure, including the apparent dispersion of the RIXS band, by selective excitation of vibrational levels in the intermediate state. The suppression of low-frequency vibrational excitations at increasing detuning is shown to arise from the scattering-duration-time effect. For the quasi-dissociative 4a1 core-excited state, time-dependent wavepacket calculations predict an extended vibrational progression associated with N–H bond elongation. These results demonstrate the importance of multimode nuclear dynamics in molecular RIXS and provide a framework for interpreting high-resolution vibrationally resolved X-ray spectra.