The luminescence spectra of Pr³⁺ in Na₅La(MO₄)₄ (M=Mo, W) at 4.2 K have been investigated. One- and two-phonon vibronic replicas are observed in the excitation spectra, suggesting that the contribution of the Delta process (Franck-Condon replicas) to the vibronic coupling strength of Pr³⁺ is significant. The transition probability A_vib for the vibronic lines in the ³H₄ to ³P₀ excitation spectra of Pr³⁺ in several isostructural (scheelite-like) host lattices scales with the transition probability for the zero-phonon line. This is consistent with a vibronic coupling in which the Delta process dominates. The transition probabilities A_vib for the ³H₄ to ³P₂ excitation and the ³P₀ to ³F₂ emission transitions, and the vibronic spectra of Pr³⁺ in Na₅La(MO₄)₄ give additional support for this conclusion.