The uplift capacity of suction bucket foundations in soft clay is commonly improved by enlarging or structurally modifying the bucket, whereas direct strengthening of the soil plug has received little attention. This study proposes in-bucket vacuum preloading as an alternative method that exploits the sealed underwater space of the bucket to consolidate the enclosed soil. A double-layer bucket was developed for proof-of-concept testing, a consolidation model was formulated under combined vacuum pressure and seabed seepage boundary conditions. The governing equations were solved using the finite difference method and evaluated against finite-element simulations and model-test. Results show that vacuum loading produces greater pore pressure dissipation and consolidation within the soil plug than outside the bucket wall. The strength gain is strongly depth dependent and greatest near the bucket end, where enhanced base resistance governs the uplift capacity improvement. When consolidation degree reaches 90%, the uplift capacity reaches about 1.34 times the untreated capacity. Parametric analyses show that smaller bucket aspect ratios accelerate consolidation over a fixed treatment period, whereas higher vacuum pressures increase the attainable capacity gain. Meaningful improvement remains achievable when vacuum pressure loss is considered, demonstrating the potential of in-bucket soil improvement for suction bucket foundations in soft seabeds.