Mott insulators with spin-orbit entangled j=1/2 moments host intriguing magnetic properties. The j=1/2 wave function requires cubic symmetry, while a noncubic crystal field mixes j=1/2 and 3/2 character. Spectroscopic studies of 5d^5 iridates typically claim noncubic symmetry, e.g., based on a splitting of the excited j=3/2 quartet. A sizable splitting is particularly puzzling in antifluorite-type K2IrCl6, a frustrated fcc quantum magnet with global cubic symmetry. It raises the fundamental question about the stability of j=1/2 moments against magneto-elastic coupling. Combining resonant inelastic x-ray scattering with optical spectroscopy, we demonstrate that the multi-peak line shape in K2IrCl6 reflects a vibronic character of the j=3/2 states rather than a noncubic crystal field. The quasimolecular crystal structure with well separated IrCl6 octahedra explains the existence of well-defined sidebands that are usually smeared out in solids. Our results highlight the spin-orbital-lattice entangled character of cubic K2IrCl6 with ideal j=1/2 moments.