Ultrasonic Vibration–Assisted Friction Stir Welding (UVaFSW) represents a next-generation solid-state joining technology designed to overcome the limitations of conventional friction stir welding by integrating ultrasonic energy to promote refined microstructures and superior joint integrity. This study presents a systematic optimisation and metallurgical investigation of UVaFSW applied to aerospace-grade AA2060-T8E30 Al–Li alloy, linking process design, microstructural evolution, and mechanical performance through Response Surface Methodology (RSM). A Central Composite Design was employed to model the effects of tool rotational speed (TRS), traverse speed (TTS), axial force (AF), tilt angle (TA), and ultrasonic vibration amplitude (Amplitude, µm) on ultimate tensile strength (UTS), elongation (EL), and hardness (HV). The optimised parameters (TRS=1200 rpm, TTS=2 mm s⁻¹, AF=6 kN, TA=2°, Amplitude=15 μm) yielded UTS=449.8 MPa, EL=12.9%, and HV=129.1, achieving~90% joint efficiency with R² > 94%. Metallurgical characterisation revealed that an intermediate-amplitude (15 μm) deformation induced intense dynamic recrystallisation, refined grains to ≈4.6 μm, and homogenised the S′–Al₂CuMg and Al₆(Mn, Fe) precipitates, simultaneously enhancing strength and ductility. This work establishes a quantitative link between ultrasonic amplitude, microstructural evolution, and mechanical optimisation in Al–Li alloy UVaFSW, demonstrating its potential to produce defect-free, high-efficiency joints for sustainable aerospace manufacturing.