A formulation for the analysis of the nonlinear vibrations of Variable Stiffness (VS) plates is presented. Third-order Shear Deformation Theory (TSDT) is employed in conjunction with a mixed variational formulation. The solution is sought via Ritz approximation for the spatial dependency, while time dependency is handled via Differential Quadrature (DQ) and Harmonic Balance (HB) methods. The main advantage of the framework is the reduced computational time, which is of particular interest to explore the large design space offered by variable stiffness configurations. The results are validated against reference solutions from the literature. Exemplary parametric studies are presented to demonstrate the potential of the approach as a powerful means for exploring the nonlinear vibration response of VS plates.