We quantitatively study the Raman and photoluminescence (PL) emission from single-layer molybdenum disulfide (MoS₂) on dielectric (SiO₂, hexagonal boron nitride, mica and the polymeric dielectric Gel-Film^®) and conducting substrates (Au and few-layer graphene). We find that the substrate can affect the Raman and PL emission in a twofold manner. First, the absorption and emission intensities are strongly modulated by the constructive/destructive interference within the different substrates. Second, the position of the A_1g Raman mode peak and the spectral weight between neutral and charged excitons in the PL spectra are modified by the substrate. We attribute this effect to substrate-induced changes in the doping level and in the decay rates of the excitonic transitions. Our results provide a method to quantitatively study the Raman and PL emission from MoS₂-based vertical heterostructures and represent the first step in ad hoc tuning the PL emission of 1L MoS₂ by selecting the proper substrate.