The application of a virtual synchronous generator (VSG) to provide virtual inertia in isolated microgrids has emerged as a promising control strategy for converter-interfaced renewable energy sources. However, tuning the VSG parameters requires an accurate characterization of frequency dynamics, which remains a challenge, particularly in hybrid microgrids combining conventional and renewable units. In this context, this paper proposes a reduced-order model to support the parameter tuning of VSGs in an isolated hybrid microgrid composed of an oil and gas facility powered by gas generators connected to an offshore wind turbine. A VSG-based control strategy is applied on the grid-side converter of the wind turbine, allowing it to contribute to frequency regulation and inertia emulation. An analytical formulation is developed to determine the frequency nadir, its time of occurrence, and the rate of change of frequency based on the fixed parameters of the gas generators and the tunable parameters of the VSG. A procedure for parameter tuning to attain the desired frequency dynamics is derived from the analytical formulation. Simulations based on MATLAB/Simulink Simscape Electrical model demonstrate the effectiveness of the proposed procedure by illustrating its consistency in guiding parameter tuning and achieving the desired frequency dynamics.