Density functional theory calculations of structural, phonon, thermal, electronic, and optical properties of V <i>x</i> Mo1− <i>x</i> S2 <i>z</i> Se2(1− <i>z</i> ): ( <i>x</i> = 12.5% and <i>z</i> = 0%, 6.25%, 12.5%, 18.75%)

First-principles calculations were employed to investigate the effects of dilute vanadium (V) and sulfur (S) doping on the structural, phonon, thermal, electronic, and optical properties of VxMo1−xS2zSe2(1−z) alloys derived from two-layer hexagonal molybdenum diselenide (2H–MoSe2). The results indicate that all considered compositions preserve structural integrity with only slight lattice contraction and remain dynamically stable. Increasing sulfur concentration enhances lattice stiffness, Debye temperature, and overall thermal stability while simultaneously reducing lattice thermal conductivity. Electronic structure calculations reveal that the systems maintain an indirect semiconducting nature with tunable and moderately increased bandgaps as the sulfur doping concentrations increases, accompanied by an enhanced dielectric response. Thermodynamic analysis based on Helmholtz free energy suggests that alloys with moderate sulfur content exhibit greater thermodynamic stability, whereas higher sulfur substitution slightly decreases stability. Furthermore, the observed reduction in heat capacity and entropy at higher sulfur concentrations indicates stronger phonon scattering and suppressed lattice heat transport. Overall, V/S co-doping emerges as an effective strategy for tuning the multifunctional properties of two-layer 2H–MoSe2. Moderate sulfur concentrations are favorable for stable electronic and optoelectronic applications, while higher sulfur contents may be advantageous for thermal management technologies.