Rare-earth-doped oxide phosphors are important for solid-state lighting, and photonic applications because their spectroscopic properties are strongly governed by the interplay between local structure, lattice vibrations, electronic structure, and crystal-field (CF) interactions. We present a comprehensive investigation of the structural, electronic, vibrational and spectroscopic properties of R3+ (R = Dy, Eu)-doped CaAl4O7 phosphors using complementary first-principles density functional theory (DFT) computations and CF parameters (CFPs) modelling approaches. Electronic-structure computations were performed within DFT employing the LDA+U and LDA–1/2 formalisms, while lattice vibrations were analysed using both DFT-LDA computations and the semi-empirical GF-matrix force-field methods. The DFT geometry-optimized structures preserve the monoclinic space group C2/c while introducing local distortions around the substitutional Ca2+ sites. The computed electronic structures reproduce the experimentally observed wide band-gap features and reveal distinct differences between Dy3+- and Eu3+-doped systems. In Dy3+-doped CaAl4O7, the 4f states remain well separated from the band edges, whereas in the Eu3+-doped system, Eu(4f) states appear within the band gap, associated with O(2p) → Eu(4f) charge-transfer transitions. The calculated Raman- and infrared-active phonons agree well with experiment. The modelled CFPs successfully reproduce the observed Stark splitting patterns and 4f–4f transition energies of Dy3+ and Eu3+ ions. The standardized monoclinic CFP sets and rotational invariants reveal stronger CF for Dy3+ than for Eu3+, correlating with differences in local coordination distortions. Overall, the present study establishes structure-property relationships by demonstrating how local coordination distortions induced by rare-earth substitution govern the electronic structure, lattice dynamics, crystal-field strength, and the resulting spectroscopic properties of Dy3+- and Eu3+-doped CaAl4O7 phosphors.