A theoretical investigation of the vibrational levels of the electronic ground state (X(2)A(2)") of NH3+ is reported. Two representations of the potential energy surface are investigated. The first one, in normal coordinates, is based on an improvement of the interpolated potential energy surface (PES) given by Leonard et al. (C. Leonard, S. Carter, N. C. Handy and P. J. Knowles, Mol. Phys., 2001, 99, 1335). It is used in conjunction with our variational code MULTIMODE. The second is a full six-dimensional fitted potential energy function (PEF) in internal coordinates obtained from many ab initio calculations. This potential is used in conjunction with both MULTIMODE, and our new variational code expressed in internal coordinates for the determination of the exact vibrational levels of ammonia-like molecules. From a comparison of the two sets of results, we assess the possibilities of investigating larger systems using interpolated PESs together with the M-mode approximation within MULTIMODE.